Grid responsive control device

ABSTRACT

A load control device which is responsive to a physical variable representing the balance between load and generation on an electricity grid. The control device varies the energy consumption of the load based on the current value of the physical variable of the grid relative to a central value of that physical variable, which is derived from past readings of the physical variable of the grid. The grid responsive control device also takes into account the time since the load last varied its energy consumption in determining whether or not the grid variable load control should be provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending U.S. patent applicationSer. No. 11/921,362 filed Nov. 30, 2007, which is the national stage ofInternational Application No. PCT/EP2006/005252 filed Jun. 1, 2006.

FIELD OF THE INVENTION

The present invention relates to a means and method for controlling thebalance between supply and generation on an electricity grid.

BACKGROUND OF THE INVENTION

A reliable source of electricity is essential for almost all aspects ofmodern life.

Providing reliable electricity is, at present, an enormously complextechnical challenge. It involves real-time assessment and control of anelectricity system consisting of generation, of all types (nuclear,coal, oil, natural gas, hydro power, geothermal, photovoltaic, etc.),and load e.g. the appliances, instruments etc. using the electricity.

The electricity is supplied over a distribution network consisting oftransmission lines interconnected by switching stations. The generatedelectricity is generally ‘stepped-up’ by transformers to high voltages(230-765 kV) to reduce transmission losses of electricity (throughheating). The generators, distribution networks and loads comprise anelectricity power grid.

Reliable operation of a power grid is complex as, at present,electricity must be produced the instant it is used, meaning powergeneration and demand must be balanced continuously. In existing powermanagement systems, the supply of electricity is balanced to demand byplanning, controlling and coordinating the generation of electricity.

Failure to match generation to demand causes the frequency of an ACpower system to increase when generation exceeds demand and decreasewhen generation is less than demand.

In the UK, the electricity boards must maintain a nominal frequency of50 Hz and are allowed a variation of ±½%. In the US, this nominalfrequency is 60 Hz. In some closed loop systems, such as an airplane,the nominal frequency is 400 Hz. The nominal frequency is the frequencyof the AC power that a grid was designed for and it is intended to keepthis frequency controlled and stable.

Random, small variations in frequency are normal, as loads come on andoff and generators modify their output to follow the demand changes.However, large deviations in frequency can cause the rotational speed ofgenerators to move beyond tolerance limits, which can damage generatorturbines and other equipment.

The variation in frequency can also damage loads.

A frequency change of just ±½% is a large signal in terms of theprecision of modern semiconductor instrumentation.

There are problems with the present supply-side style architecture ofmatching generation to demand. At present, when extreme low-frequenciescan not be dealt with, i.e. demand out-strips generation, automaticunder-frequency load shedding may be triggered, which takes blocks ofcustomers off-line in order to prevent a total collapse of theelectricity system. This may have the effect of stabilizing the system,but is extremely inconvenient and even hazardous to the user.

After a blackout the grid is at a particularly sensitive stage andrecovery is slow. Large generators generally require other generators tomake some power available to start or re-start it. If no power isavailable, such generator(s) cannot start. So grid systems haveservices, known as “black start” services, whereby a subset ofgeneration has the capacity to start and continue generating, even whenthe rest of the grid is inactive (i.e. black). Grid operators haveprepared planned sequences for restoring generation and load. Theseensure that the limited initial supplies are used first to providecommunication and control, then to start up bigger generators, andthereafter the load is progressively connected to match the increasingavailability of generation.

The entire process of black start is a fraught one. A blackout is a veryrare event, and not one that can be practiced except in an actualcrisis. Everybody involved is under severe pressure, and the systems arebeing operated quite outside their normal operating range (and sometimesoutside their design range). Every step when load or generation is addedis a shock to the system and the grid can take seconds or minutes tostabilize after it happens. Prudence would suggest making changes insmall increments. This inevitably slows down the overall process,prolonging the blackout for those who are still to be reconnected.

In order to insure as much as possible against load-shedding, a powersystem will be operated at all times to be able to cope with the loss ofthe most important generator or transmission facility (i.e. the mostsignificant single contingency). Thus, the grid is normally beingoperated well below its capacity such that a large random failure doesnot jeopardize the system as a whole. This, however, means that thegeneration is not operating as efficiently as possible, with a resultingincrease in electricity supply costs.

High air conditioning and other cooling loads in the summer and highspace heating loads in the winter are a normal cause of peak-loads. Gridoperators, however, use rigorous planning and operating studies,including long-term assessments, year-ahead, season-ahead, week-ahead,day-ahead, hour-ahead and real time operational contingency analyses toanticipate problems.

The unexpected can still occur, which is why the system operates withheadroom for compensating for the largest contingency. Utilities can useadditional peaking generators, which have high running cost, to provideadditional electricity when needed or, alternatively do not operate maingenerators at capacity so as to leave some potential for extrageneration to satisfy excess loads. Both of these methods result in ahigher unit cost of electricity than if the system was operating nearerto capacity.

There has been proposed an alternative architecture for matching loadand generation to that presently used. The general idea is to compensatefor differences between load and generation using the demand-side by wayof load management.

Limited literature exists on the concept of using load, or demand, tocontribute (at least) to grid stability.

U.S. Pat. No. 4,317,049 (Schweppe et al.) proposes such a differentbasic philosophy to existing electric power management in which bothsupply and demand of electricity respond to each other and try tomaintain a state of equilibrium.

This document identifies two classes of usage devices. The first typeare energy type usage devices characterized by a need for a certainamount of energy over a period of time in order to fulfill theirfunction and an indifference to the exact time at which the energy isfurnished. Examples were space conditioning apparatus, water heaters,refrigerators, air compressors, pumps, etc. The second class was a powertype usage device characterized by needing power at a specific time.Such devices would not be able to (fully) fulfill their function if thepower was not supplied at a designated time and rate. Examples includelighting, computers, TVs, etc.

The Frequency Adaptive, Power-Energy Re-scheduler (FAPER) of theSchweppe et al. patent provided its power management by application of aFAPER to energy type usage devices. The Schweppe et al. patentparticularly discusses application of the FAPER to a water pump forpumping water into a storage tank.

The water level in the water tank has a minimum allowable level Ymin anda maximum allowable level Ymax. Ordinarily, the water pump will beswitched on to pump water into the storage tank when the level falls toor below the minimum level and turns the pump off when the maximum levelis reached. Otherwise, the pump is idle.

The FAPER modifies these limits (Ymax, Ymin) depending upon the systemfrequency. Thus, in a period of high frequency (electricity demandshortage), i.e. when the grid frequency increases above nominal, theminimum water level causing the pump to activate (Ymin) is increased andthe maximum water level (Ymax) is also raised. Thus, the pump switcheson at a higher level and stops at a higher level than operation notunder the control of a FAPER. This means that the excess in generationis being taken up. Using the same principle, as the electricityfrequency drops below the grid nominal frequency (a generationshortage), the minimum and maximum water levels are lowered. Thislowering results in ON pumps being switched off sooner and OFF pumpscoming on later than usual, and so using less electricity, therebyreducing the load.

According to Schweppe, the raising of the limits (particularly themaximum) and the lowering of the limits (particularly the minimum)should have an extent cap, defined by either user desires or safetyrequirements. Thus, the limits should be extendable, but only to acertain extent, as otherwise the tank could unacceptably empty oroverflow.

The broad concept uncovered by Schweppe in this patent is that consumingdevices, which incorporate some sort of energy store and operate to aduty cycle, are useful in providing grid responsive behavior. Whenrunning, the energy store is being replenished or filled and, thus, thepotential energy of the store is increasing. When the devices are notrunning, their function is preserved because of the load's ability tostore energy.

The FAPER modifies the timing of the load's consumption, withoutdetriment to the service provided by the device, using the gridfrequency as a guide. Thus, the potential energy of the device isincreased when the grid frequency is high in order to maximize theamount of energy fed into the device which is stored. This compensatesfor any excess. During times of insufficient generation (highfrequency), the potential energy of the device is lowered, therebyreleasing energy to the grid and compensating for the shortage.

Moving on from the FAPER, a different and improved “responsive loadsystem” was disclosed in GB 2361118 by the present inventor. Theresponsive load system was based on the same underlying principle as theFAPER devices, that grid stability can be at least aided by usingdemand-side grid response, and built on the response method and offereda further enhancement of using probabilistic methods as to the ON/OFFswitching timing for the load.

One problem with the FAPER device is that, without any randomization,the smallest movement of the frequency could result in all loads withFAPERs applied responding in the same way and doing so simultaneously.This could result in a destabilizing influence on the grid. A moregradual response is needed and the responsive load system offered thisby distributing the frequencies to which each device is responsive byusing a randomized function.

As mentioned above, the responsive load system of GB 2361118 defines aprobability based method for choosing the frequency to which a device issensitive. In this way, a progressively larger proportion of theresponsive load device population changes the load as the systemfrequency departs from the nominal frequency of the grid.

In more detail, the responsive load system uses a randomizer to chooseboth a high frequency and a low frequency to which the device issensitive. This is advantageous over the FAPER device as more and moreload is switched on or off progressively as the frequency increases ordecreases, respectively.

The random inputs for the high and low frequencies to which the devicesare sensitive are revised from time to time. This step has the advantageof distributing any disadvantages of responsive devices among thepopulation and ensuring that no one device was stuck with unfavorablefrequency triggers. For example, it would not be appropriate if aparticular device was constantly sensitive to the slightest change infrequency whereas another device had such broad trigger frequencies thatit only provided frequency response in extreme grid stress situations.

One problem with this system is that the controller is not tamperproof.Users, such as users of air conditioning, might choose to turn up theircontrols because of the slight heating/cooling of a room beyond thatdesired as a result of a frequency responsive load change being noticed.Thus, if the air conditioner is generating in a lower temperature range,that is the air conditioner is working harder and is on more frequently,because of an increase in grid frequency, and a user notices this andturns the air conditioning down, before the frequency returns to anacceptable level, then the response has been lost.

Partially because of the above problem, the Grid Stability System of UKpatent application number 0322278.3 was formed. The grid stabilitysystem prevents an end user from overriding the frequency responsefunction by fixing the frequency triggers at pre-grid stress settings.In this way, manipulation of a set point controller, such as athermostat, is made ineffective for the duration of the period of highstress.

The grid stability system also defines three states of the system,normal, stress and crisis. The stress level of the grid determines whichof the above three grid states are relevant.

The stress level of the grid can be determined by comparing the presentgrid frequency to limit values for the frequency and determining whetherthe current frequency falls inside limits chosen to represent a normalstate, a stressed state or a crisis state.

Rapid changes in frequency, whatever their absolute value, are also usedas indicators of grid stress level by defining limits for the rate ofchange of the grid frequency.

The grid stress level can also be indicated by an integration, overtime, of the deviation of the grid frequency from the nominal gridfrequency. Thus, even if the extent of frequency departure is verysmall, if it departs for a long enough time, then a grid stress orcrisis condition is still determined.

The grid status is, therefore, determined, according to the gridstability system, by taking into account instantaneous large frequencydepartures from nominal, rapid changes of frequency and accumulativelylarge, but not necessarily outside a preferred frequency change at anygiven time, departures all being signs of grid stress. Each of thesepossible types of grid indicators has an associated set of limits thatindividually or in combination determine whether the grid is in a normalstate, a stressed state or a crisis state.

Having determined the status of the grid, that is whether the grid is ina normal state, a stressed state or a crisis state, the controller ofthe grid stabilizing system adapts its grid responsive behaviordepending upon the determined grid status. If a normal status isdetermined, the device provides response to frequency changes in thesame way as the original responsive load device. So, as grid frequencyrises above the temperature determining trigger frequency, off deviceswill switch on in order to “take up” the extra generation. In the casethat the grid frequency falls below a low frequency trigger value, “on”devices will switch off to reduce the load upon the grid.

If operated according to the FAPER invention, a physical variableassociated with the load (water level, temperature) is still controlledwithin minimum and maximum values during this time, but the limits areextended so that devices switched on and devices already on will stay onfor longer than if the controlled device was operating within the normalfrequency limits. Similarly, in periods of overly high grid frequency,off devices will stay off for longer as the lower limit of the physicalvariable has been extended as well.

Using the example of the water tank, as grid frequency increases abovethe higher frequency limits, off devices will switch on and on deviceswill stay on until the physical variable reaches its extended limit oruntil the frequency returns below the higher frequency limits. If thenormal range for the water tank depth lies between 1 and 1.5 meters, forexample, if the grid frequency rises above the higher frequency limits,off devices will switch on and on devices will stay on up to an extendedwater depth of 1.7 meters, for example. Thus, the potential maximumlevel of the water tank has been raised above its normal level. Further,the potential energy of a population of water pumps controlled in thisway will have increased their average depth of water. This serves tocompensate the excessive generation, which produced the high gridfrequency, and stored the excessive grid energy, which will compensate,to some extent, the higher frequency. When the frequency drops below thelower frequency limits, this energy is repaid to the grid by switchingon devices off and keeping off devices off up to a lower extendedphysical variable limit of, for example, 0.8 meters. This allows a largepopulation of devices to reduce their potential energy and supply theenergy difference into the grid. This serves to compensate for the lackof generation that resulted in the low frequency.

If operated according to the responsive load system of GB2361118, thecontrol limits remained unchanged, but the device could be switched onor off if the system frequency moved beyond the frequency to which thedevice was sensitive. So the device could be switched before it reachedits control limits, and this extra switching modified the load and socontributed the change of load necessary to balance the system.

Using the example of the water tank again, low frequency would cause anon device to switch off at, for example, 1.4 m and so earlier than ifthe limit of 1.5 m was reached, and, conversely, high frequency wouldcause the device to switch on at, for example, 1.1 m and so earlier thanif the lower limit of 1 meter was reached.

Together, these cause the average water level in a population of devicesto become lower when the frequency is low, and to become higher when thefrequency was high, although each individual device would operate withinits control limits.

The frequency limits for a particular device are chosen to fall withinan upper frequency range and a lower frequency range. As with theResponsive Load previously discussed, a randomizer is used to choose theparticular high trigger frequency and the particular low triggerfrequency such that a population of devices have high triggerfrequencies and low trigger frequencies distributed within the upperfrequency range and the lower frequency range, respectively. Thus, awindow is provided between the distribution of high trigger frequenciesand low trigger frequencies. This window is centered around the nominalfrequency. The window allows the controlled load, e.g. a water tank,refrigerator or air conditioner, to operate entirely as normal, i.e. asthough it did not have a frequency responsive controller applied to it,when the frequency of the grid is close enough to the nominal gridfrequency to lie within the window. Response is provided only when thegrid frequency extends outside this window.

In the case that a stressed state is determined, the control limits ofthe device are frozen at pre-stress settings so that manipulation of acontrol panel to adjust a set point for the sensed physical variable(e.g. temperature) is ineffective. Thus, the user of the controlled loadcannot adjust the loads settings, for example by using a thermostatcontrol. If the responsive device is controlling an air conditioner, agrid responsive induced change in room temperature could be noticed. Auser may decide to attempt to counter the change in temperature byadjusting the thermostat. The responsive load device of the gridstabilizing system overrides such an adjustment of the set point whenthe grid is determined to be in a stressed condition. This is importantas the grid is particularly sensitive during a period of grid stress andusers negating the response provided could worsen the destabilization ofthe grid.

In extreme circumstances, when a risk of blackout exists, a grid crisisstate may be determined. In the grid crisis state, the grid stabilizingsystem relaxes the control of the physical variable limits and allowsthem to move outside of a preferred range. In a high frequency gridstate, the loads are switched on until the grid crisis state is exitedand in a low frequency grid crisis state, the responsive load (e.g.fridge) is switched off until the crisis state is exited. The switchingon and switching off is carried out irrespective of the control limits,so a fridge, for example, could continuously cool down to well below apreferred minimum or the fridge could be allowed to warm up to anambient condition well above a preferred maximum temperature. Theseextreme measures are only taken in the most serious of grid conditions,when the alternative is a blackout.

Modeling of the prior art frequency and responsive control devices hasuncovered previously unknown problems with the above described prior artgrid responsive loads.

It has been found that after response has been affected for a period oftime, a population of the devices will tend to approach the physicalvariable control limits, and start switching at an excessive rate. Forexample, a refrigeration unit controlled by a frequency responsivedevice will reach its extended temperature limits after a sustainedperiod of high or low frequency. Using the example of a higher thannominal grid frequency, devices will switch on until the low temperaturelimit has been reached and will then switch off, but as soon as thetemperature passes back over the low temperature limit the device willagain check whether the grid frequency is above its higher frequencylimits, and if so will switch on again immediately. This results in veryfrequent switching as the device is attempting to provide frequencyresponse to a unit close to its physical variable limits. This is notdesired behavior as it could damage the controlled loads. Excessiveoscillating on and off switching of the load will reduce the lifespan ofthe device.

Also, modeling of the prior grid frequency responsive loads have beenfound to have an unexpected effect on the grid frequency. It was assumedthat the responsive devices would smooth the grid frequency to provide afar clearer, less noisy, grid frequency. This did not, however, entirelybear out during modeling, and some previously unknown strange behaviorof the grid frequency was observed as a result of the responsive loads.

The prior art grid responsive control devices do not provide any specialassistance to a grid recovering from a blackout, but the stabilizingeffect of responsive loads are needed more than ever at this time.

Amongst other objects, the present invention aims to have an improvedstabilizing effect on a power grid.

The present invention also aims to reduce the switching of powering ofan energy store during operation of a grid responsive device controllingthe powering of the energy store.

The present invention also aims to aid the grid start-up after ablackout. In particular, the present invention aims to soften the shocksto the system during the black start process. The loads and generatorscan be reconnected more quickly, so speeding recovery.

The device of the present invention also aims to overcome the aboveidentified problems with prior art grid responsive control devices.

SUMMARY OF INVENTION

According to a first aspect, the present invention provides a controldevice for controlling an energy consumption of a load on an electricitygrid, said control device comprising:

means for sensing over a period of time values of a physical variable ofthe grid, said physical variable varying in dependence on a relationshipbetween electricity generation and load on the grid;

means for determining a central value of the physical variable of thegrid from said values of the physical variable of the grid; and

means for varying the energy consumption of said load, said varyingdependent upon said central value.

According to a second aspect, the present invention provides acorresponding method of controlling an energy consumption of a load onan electricity grid

Conventionally, a nominal frequency of the grid and a current value ofthe physical variable is used for controlling the energy consumption ofthe load. The present invention, however, uses some function of the pastreadings of the physical variable. This gives a long-term past value forthe central value and it is this central value that is taken intoaccount for controlling the energy consumption of the load. Modeling ofthe present invention has shown the use of a historically based centralvalue for controlling the energy consumption of the load removes thestrange effects on the grid frequency found to exist with prior art gridresponsive control devices.

The first and second aspects of the present invention can be used incombination with prior art grid responsive control devices as discussedabove. Alternatively, the preferred form of the present inventionencompasses a comprehensive grid responsive control device combinablewith any of the below described other aspects of the invention or any ofthe below described preferred features.

In a third aspect, the present invention provides a control device forcontrolling the energy consumption of a load in an electricity grid,said control device comprising:

means for sensing a value of a physical variable of the grid, saidphysical variable varying in dependence on a relationship betweenelectricity generation and load on the grid;

means for sensing a value of a physical variable of the load, saidphysical variable of the load representative of the energy stored by theload;

means for varying the energy consumption of said load when a value ofsaid physical variable of the grid reaches a trigger value; and

means for determining the trigger value, said determining of the triggervalue dependent upon said sensed physical variable of the load.

In a fourth aspect, the present invention provides a correspondingmethod of controlling an energy consumption of a load on an electricitygrid.

The third and fourth aspects of the present invention control the loadbased upon a value of the grid variable, which is selected in dependenceupon the variable of the load. Thus, these aspects of the inventionallow the energy consumption of the loads to be changed in a way thatvaries with the sensed physical variable of the load. By taking intoaccount the variable of the load in this way, the energy consumption ofthe load can be controlled to minimise the rate of changes in the energyconsumption of the load. This is so because loads closer to theirnatural switching points (which is determined by the variable of theload) are favoured for grid responsive control.

Again, the invention provided by the third and fourth aspects areadvantageous when used in combination with prior art grid responsivecontrol devices. These aspects are especially advantageous when combinedwith the first and second aspects of the invention described above andprovide further advantages when combined with the preferred embodimentsdetailed below.

The preferred embodiments described below are applicable as preferredembodiments of the methods of the present invention or the apparatus.Thus, the features of the preferred embodiments may be adapted toinclude the means of a control device for performing the feature or maybe adapted to comprise method steps. The preferred features aregenerally worded in apparatus terms, but are applicable to all aspectsof the present invention.

In a preferred embodiment of the first and second aspects of theinvention, the control device is adapted to determine a trigger value ofthe physical variable of the grid based upon said central value and tovary the energy consumption of the load when a current value of thesensed physical variable of the grid reaches the trigger value.

The control device may determine a trigger value based upon just thesensed physical variable of the load or both the sensed physicalvariable of the load and the sensed physical variable of the grid, orjust the sensed physical variable of the grid. This combination offeatures of the present invention is advantageous as set out in moredetail below.

According to a preferred form of the aspects of the invention, the meansfor determining the trigger value comprises a function for randomlyproviding the trigger value between a determined upper or lower value ofthe physical variable of the grid and the central value.

The control device may also preferably be adapted to generate a randomvalue and to determine the trigger value based further upon said randomvalue and to control the energy consumption of the load dependent uponthe trigger value.

Thus, all aspects of the present invention may advantageously utilize arandom value in determining the trigger value as this will provide arandomized element to the trigger value, meaning that a population ofloads controlled in this way will not all change their energyconsumption in a synchronized way, which would destabilize the grid.

According to a further preferred feature, the control of the energyconsumption of the loads is performed by comparing the trigger value ofthe physical variable of the grid with the current sensed physicalvariable of the grid.

In a preferred embodiment, the physical variable of the grid is afrequency and so it is the frequency of the grid which is sensed.Alternatively, an amplitude of the supply voltage could be sensed, whichalso shows dependence upon the balance between generation and load ofthe grid.

Thus, according to one preferred embodiment of the present invention acentral frequency is determined from past readings of the frequency ofthe grid and the control device tends to resist any change in frequency,up or down, to some extent regardless of the absolute frequency of thegrid. Thus, while in prior art grid frequency responsive control devicesit is the nominal frequency of the grid which is used as a referencepoint for determining whether to provide response, the presentinvention, differs in using a historical value, around which theresponse trigger frequency is set.

The basic concept is that even during a period in which the frequencydrops below nominal, if the frequency starts to rise, then theresponsive control device will function to resist this change, despitethe frequency actually moving closer to nominal, which conventionallywas considered favorable.

During periods of low frequency, the average input energy in apopulation of loads drops in order to reduce the energy extraction fromthe grid and, therefore, compensates for the excessive load causing thefrequency drop. Energy is, in effect, being loaned to the grid.

Ideal behavior would be to recover this energy, and restore the totalenergy store, before the frequency again returns to the nominal gridfrequency. So a frequency rising from a below nominal value is the mostfavored time to repay the energy to the grid.

Similarly, but symmetrically, during a period of above nominal frequencyon the grid, the loads are controlled to borrow energy from the grid inorder to take up some of the excess generation. The favored behavior isto return this energy before the frequency again reaches the gridnominal frequency.

The behavior of the control device reinforces the natural emergentproperty of grids by which the frequency is an indication of excesses ordeficits of energy in the grid. If the frequency is low, there is anenergy deficit, and if high, there is a surplus. If the deficit orsurplus is largely absorbed by the loads, then the frequency signal ismade clearer.

The central value of the variable, e.g. frequency, is preferablyprovided by calculating a moving average from past readings of thephysical variable of the grid.

The trigger value is a value, e.g. frequency, at which responsivecontrol devices will either increase or decrease their energyconsumption and is determined based upon this central value. Thus, forexample, for a population of such control devices, if the currentfrequency is above the central frequency, the energy consumption of theload will tend to increase, and if below, the energy consumption of theload will tend to decrease.

A random element is also preferably included in the determination of thetrigger value to ensure that the increasing or decreasing of loads isgradual so as to not burden the grid with a population of loads allswitching at the same time, thereby negating the stabilization object ofthe control device. Thus, large scale synchronized switching is avoided.

The overriding effect of the use of the central value to determine thetrigger value, at which the energy consumption of the load is changed,means that a population of loads controlled by such devices, activelyand continuously damp grid frequency variations.

In a preferred embodiment, the device is further adapted to: sense aphysical variable associated with the load; determine upper and lowerlimits for the physical variable associated with the load; and changethe energy consumption of the load when the physical variable associatedwith the load reaches its upper or lower limits.

This feature ensures the load still performs its main function, which isto maintain a variable associated with the load within certain limits.These limits may be derived from a user selection. For example, the setpoint of a thermostat for air-conditioning or a refrigerator settingwould lead to limits being defined. The temperature of the space beingconditioned or refrigerated should not exceed or go outside of theselimits. The temperature is kept around a desired temperature. Arefrigerator, for example, would operate to a duty cycle such that whenthe temperature reaches its upper limits, the cooling mechanism of therefrigerator will be switched on so as to lower the temperature. Ofcourse, once the temperature reaches its lower limits, the refrigeratorwill switch off.

The majority of the description that follows is concerned with the loadsthat control the physical variable of the load within the control limitsby turning the energy consumption on or off. However, loads in whichthis control is achieved by increasing or decreasing the energycontinuously are also applicable with the control device of the claimedinvention.

The preferred embodiment provides two layers of control, the first is toincrease or decrease the energy consumption of the load to keep thephysical variable associated with the load within its control limits andthe second layer is to further control the energy consumption of theload depending upon relative rises or falls of the grid variable from acentral value.

As described above, one of the problems with prior art grid responsivedevices was that this two layer control tended to increase switchingrates after a prolonged frequency deviation. The present invention aimsto combat this switching rate increase and the third and fourth aspectsof the present invention, and preferred embodiments of the first andsecond aspects of the invention, are directed to encompass theachievement of this objective.

In a preferred embodiment, this objective is also achieved by thetrigger value (or trigger frequency) being based upon the sensedphysical variable of the load. In a preferred form, the means fordetermining the trigger value is configured to determine the triggervalue in dependence on the sensed physical variable of the load and thecontrol limits so as to reduce the rate of variation of the load.

In another preferred form, the means for determining the trigger valuecomprises a function which returns the trigger value in dependence uponthe sensed physical variable of the load, said function defining atrigger value profile varying with said physical variable of the load,said profile such that the more recently the energy consumption of theload has varied, the further the trigger value is from a central valueof the physical variable of the grid.

More specifically, in a further preferred embodiment the provision of atrigger value (e.g. frequency) is further based upon a ratio of a valuerepresenting said sensed physical variable relative to the upper or thelower limit of the sensed physical variable associated with the load toa range between the upper limit and the lower limit.

The above defined ratio is an indication of how much energy is stored inthe load compared with the maximum or minimum defined by the controllimits. Again, in the case of a refrigerator, when the refrigerator hasbeen on for 50 percent of the on portion of the duty cycle of therefrigerator, then the sensed variable associated with the load will behalf way to its lower temperature limit or, in other words, therefrigerator is half way to its maximum input energy. In determining thetrigger frequency for the preferred embodiment, the controlling devicetakes into account how full the energy store is and, therefore, howclose it is to a natural switching point.

In an extension of this embodiment, the trigger value varies with theratio such that the likelihood of the energy consumption of the loadbeing changed increases as the ratio increases. Thus, the ratioincreases depending upon the length of time the load has been in aparticular consumption state. For example, in the case of arefrigerator, the cooling provision means being in an off state is oneparticular energy consumption state and the cooling provision meansbeing in an on state is another particular energy consumption state. Ina preferred form, a first consumption state is one in which the energystored by the load is increasing and a second consumption state is onein which the energy stored by the load is decreasing.

The ratio can be any function representative of how long the load hasbeen in a particular energy consumption state. Thus, in a preferredembodiment a ratio is defined representing the length of time a load hasbeen in a particular energy consumption state relative to a maximum timefor that state. Preferably, this representation is derived from thephysical variable associated with the load and its upper and lowerlimits.

The ratio is defined such that it will increase the longer therefrigerator is on and is also defined such that it will increase thelonger the refrigerator is switched off. If the likelihood of the energyconsumption state of the load changing increases as this ratio increasesthen the switching of the load between energy consumption states isminimized. This is, as mentioned before, important for preventing longterm damage to the load equipment, which would be unacceptable to theconsumer.

It is an important feature of preferred embodiments that the determinedtrigger value takes into account how close the load is to a naturalswitching point or how long the load has been in a particular energyconsumption state as compared to a maximum length of time as determinedby how close the physical variable associated with the load is to theloads maximum and minimum values for that variable. A refrigerator in a“cooling on” state is closer to its natural switching point as thesensed physical variable approaches a lower limit for the temperature ofthe refrigerator. Conversely, the refrigerator in a “cooling off” stateis closer to its natural switching point as the sensed physical variableapproaches an upper limit for the temperature of the refrigerationspace.

Thus, some ratio representing the sensed physical variable's relativeposition between the maximum and minimum limits for the physicalvariable associated with the load is the preferred way for determiningthe load's natural switching point. The ratio is taken into account bythe function calculating the device's trigger frequency.

In a preferred embodiment, the control device is adapted to determine anupper and a lower limit for the physical variable of the grid; whereinthe provision of a trigger value is further based upon said upper andlower limits for the physical variable of the grid. In this way, thecontrol device appropriately distributes the trigger frequency of apopulation of the devices between the upper and lower limit in order toprovide response when it is needed.

In a preferred embodiment, the value of the trigger frequency is suchthat loads remaining in a particular state for a longer time than othersare more sensitive to changes in the sensed variable of the grid byproviding an appropriate function for calculating the trigger valuebiased in this way.

More particularly, the provision of a trigger value preferably firstinvolves the control device being adapted to provide a base value of thephysical variable grid based upon said random value and said centralvalue, for example to randomly provide said base value between saidcentral value and said upper or lower control limits; the control deviceis further adapted to provide a trigger value function from said basevalue; and then determine the trigger value from the trigger valuefunction.

Thus, the randomization provided by the random value is directed to theprovision of a base value, which is, in turn, determinative of aparticular function used to provide the trigger value. In a particularlypreferred embodiment, the trigger value function defines a trigger valuevarying with the length of time that the load has been in a particularenergy consumption state. More preferably, the trigger frequency isprovided from the trigger value function varying as described above.

Thus, each device is first provided with a randomized base value, fromwhich is provided a trigger value function. The particular form of thefunction, i.e. how it varies with the ratio, is dependant upon the valueof the base value. Thus, the increase or decrease in likelihood of theload changing its energy consumption state is different depending uponthe base value.

According to the preferred embodiments of the present invention, eachcontrol device in a population will determine its own base frequency.The base frequencies will be distributed randomly across the populationso that the changing of the energy consumption of the loads or theswitching of the loads is progressive across the population.

According to the preferred embodiments, once this base frequency hasbeen determined, the exact frequency to which the device is responsivedepends upon the triggering frequency determined from the triggeringvalue function. This function is defined such that the willingness ofthe load's response varies according to its internal state. If it is ina very low energy state, and the device is on, or in a first state ofincreasing the energy stored by the load, it will not wish to switch offor to switch to a second state of decreasing the energy stored by theload except in the most extreme of grid states (as represented by thephysical variable of the grid, i.e. the frequency) but if its energystore is approaching the upper limit, it is very willing to switch offor to the second state. This changing willingness is reflected by theextent to which the trigger frequency departs from the centralfrequency.

Thus, the trigger frequency is provided with a nonlinear trajectory asthe energy state of the load varies. In order to preserve the randomdistribution of likelihood of switching across the population, the formof the trigger value function changes depending upon the randomlyprovided base value.

By changing the willingness in this way, switching will be as rare aspossible, and the switching load is distributed across the loads. Thisalso serves to maintain the diversity of the load, by avoiding buildingup a sub-population that is very close to the limits.

In a preferred embodiment the random value is provided from a randomizerconfigured to provide a distribution of base values for a population ofsaid control devices, said distribution extending from a limit of thephysical variable of the grid to the central value of the physicalvariable of the grid. This is in contrast to prior art devices where awindow is defined in which grid response is not provided and in whichthe device is allowed to behave as normal, as though it had noresponsive control device installed.

The present invention, however, distributes the population of triggervalues from a central value to a limit and, thus, response is providedat all frequencies between the determined upper and lower limits for thefrequency of the grid. In this way, borrowing of energy from the grid orrepayment of energy so borrowed from the grid occurs throughout thedetermined frequency spectrum of the grid. This is influential inproviding a damping to all movements of grid frequency from the centralfrequency.

It is also preferred that the randomizer is such that a population ofthe control devices will have trigger values having a distributionextending between the upper and lower limit of the physical variable ofthe grid. In a preferred embodiment, the trigger value varies from alimit of the physical variable of the grid to the central value as theratio moves from a minimum ratio to a maximum ratio. In this way, thetrigger value is closer to the central value the longer the load hasbeen in a particular energy state. Hence, the energy consumption of theload is more likely to change the longer the device has been in aparticular energy consumption state.

In a preferred embodiment, the change of the energy consumption of theload involves either switching the load on or switching the load off. Aload is defined as the energy consumption associated with the mainfunction of the load. For example, in the case of the refrigerator, theload is the energy consumption of the cooling provision means. Thus,using this definition, background operation of a refrigerator, such aslighting or other peripheries to the main function of the load, is notconsidered the load in the context of the specification.

It should be clear that the ratio described above is a representation ofhow long the device has been on or how long the device has been off.Preferably, the ratio is at a maximum as the sensed physical variable ofan off device approaches the device's limit associated with the offstate of a load or the ratio is defined for an on device such that it isat a maximum when the sensed physical variable approaches the limit forthe on state of the device.

In a further preferred embodiment, the provision of a trigger value isfurther based upon the particular energy state of the load (e.g. whetherthe load is an on or off state). Also preferably, the ratio representinghow close the device is to the sensed physical variable reaching alimit, is dependent upon the particular energy consumption state of theload. Thus, according to the preferred embodiments of the presentinvention, the ratio is defined differently depending upon theparticular energy state of the load (e.g. whether the load is on or offor in the first state or second state).

This is advantageous as an off load, for example, will switch onnormally at a low load variable limit (minimum stored energy). An onload, on the other hand, is approaching its natural switching point at ahigh load variable limit (maximum energy stored). It is, therefore,preferred to take the energy consumption state of a load into accountwhen defining the trigger frequency.

In yet another preferred embodiment, the upper and lower limitsassociated with the load are derived from a setpoint of the physicalvariable associated with the load. A set point could, for example, bedefined by a thermostat setting or the particular setting of arefrigerator. It is an advantageous feature of the present inventionthat not only is a good stabilizing effect achieved by providing gridfrequency response, but also that the primary function of the load, forexample, cooling, heating, pumping etc. is carried out.

There are certain grid conditions in which the limits of the sensedphysical variable associated with the load are controlled to be changedfor an extended period of time. These changing of the limits is notusually related to the provision of normal grid responsive behavior, noris it due to a change in a setpoint for the physical variable. Theextended change of the limits is more usually due to a grid condition.

According to a preferred embodiment of the invention, the upper and/orlower limit of the sensed physical variable are increased or decreasedat a rate less than or more than, respectively, a maximum rate ofincrease or decrease of the sensed physical variable of the load.

In this way, the limits are moved at a rate less than the physicalvariable could theoretically move. The lower rate of the limit movementmeans that there is still some provision for the load to be gridresponsive even while variable limits are being changed.

One example of a grid condition where this is useful is during start-upafter a power outage. As discussed above, the grid is particularlydelicate at this stage. Normally, the sensed physical variable will beoutside its normal range after a power cut and the load will need to beoperated to bring the variable back within its preferred control limits.According to a preferred aspect of the present invention, the upperand/or lower limit of the sensed physical variable is increased at arate less than a constant maximum energy consumption of the load.

Thus, there is potential during the increase in the limits to provideresponse. This ability of the load to provide grid responsive behavioris especially important during black start as the grid is especiallydelicate at this time.

In another preferred embodiment, the present invention defines a blackstart assistance mode in which a random delay is provided before theload draws energy from the grid. This preferred feature means that loadswill start drawing energy from the grid gradually, rather than them allcoming on-line at the same time and severely stressing the grid.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred forms of the present invention will be described below withreference to the following drawings.

FIGS. 1A to 1C show a preferred form of how the trigger frequency varieswith energy stored in the load.

FIG. 2 shows an example population of the loads controlled according toa preferred form of the present invention.

FIGS. 3A to 3B show an example of a profile of the trigger frequencyfunction.

FIG. 4 shows an overview of the various states of a preferred responsivecontrol device.

FIG. 5 shows a block diagram of the preferred operation of theresponsive control device of the present invention.

FIG. 6 shows a block diagram broadly outlining the operation of a PIDcontrolled load.

FIG. 6A shows a block diagram broadly outlining the operation of a setpoint adjusting grid responsive control device for a PID controlledload.

FIG. 6B shows a block diagram broadly outlining the operation of a motorpower adjusting grid responsive control device for a PID controlledload.

FIG. 7 discloses a grid responsive controlling operating with a price asan indicator of the balance between load and generation on the grid.

DETAILED DESCRIPTION OF THE INVENTION

Specific embodiments of the present invention will now be described inorder to aid in the understanding of the present invention.

The control device of the present invention is applicable to energystorage loads on a grid, which consume intermittent or variable energy.

The control device requires two main inputs, the first is a frequency ofthe grid, or another parameter representative of the balance betweenpower generation and power requirement, and the second some physicalvariable associated with the energy storage load. Generally, the primaryfunction of the load is to maintain the physical variable withinspecified control limits.

The loads will generally operate on a duty cycle of some kind, usuallywith a period in which the load is on and with a period in which theload is off. Thus, a duty cycle of 50% means that the load will be onand off for an equal amount of time. Specific loads of this kind towhich the present invention are applicable include space conditioners(e.g. heating or cooling), refrigerators and water storage pumps,amongst others.

However, modern power electronic control also makes it feasible to varythe power consumed by a motor. This can make the motor more efficient,and also means the motor is running continuously or nearly continuously,with the power varied according to the demands of the device. So in afridge, for example, the motor will reduce its power when thetemperature has reached its desired set point, will increase if thetemperature rises, and will reduce further if the fridge gets too cool.For fridges this also has some benefits in perception of noise.

The motor will generally need to operate over quite a wide power range,as, in a fridge or freezer, for example, it will have to have thecapacity to cool a warm fridge rapidly when it is switched on or when awarm mass is put into it. So there remains scope for temporarilychanging the power demands of the device from inputs other than thetemperature—such as the frequency.

The present invention provides a control device operable to vary theenergy consumed by both types of loads, i.e. by binary on/off controland by graduated or continuous increase and decrease of the energyconsumption.

For the remaining description, a refrigerator will serve as the mainexample for use with the control device of the present invention.

The present invention operates, up to a point, in common with gridresponsive control devices known from the prior art. The presentinvention utilizes the principle that energy store loads, as describedabove, can perform their function without requiring input energy fromthe grid at a specified time. Unlike lighting and other such loads,energy store loads can receive input energy at varying levels or varyingintervals and still operate in a fully satisfactory manner, providedthey are controlled so as to keep the physical variable of the loadwithin the specified control limits of the particular load.

The amounts of energy stored by the above described energy storagedevices is determined by the control limits of the physical variable. Inthe case of a refrigerator, the maximum amount of energy stored by theload is defined by the lower temperature limit for the current setpointsetting of the refrigerator and the minimum amount of energy stored isthe higher temperature limit.

In the following description, y is the normalized measurement of thephysical variable of the load being controlled by the grid responsivecontrol device of the present invention. A larger y implies more inputenergy is stored (i.e. the refrigerator is coolest) than a smaller y. Ifx represents the energy in the store, then y is a function of x, i.e.y=f(x). A normalized y can range from 0, with no energy stored, to 1, acritical maximum level of energy stored. The function is normallysufficiently close to linear to make this a useful approximation.

In the case of a refrigerator, the input energy is directed towardscooling. So y is greatest, 1, when the fridge is at its coldestpossible, and 0 when the internal temperature rises to ambient. In thecase of a tank, y is 0 when the tank is empty, and 1 at a level when thetank overflows. Normally, of course, it is controlled to within narrowerlimits, and these are referred to as the upper and lower limits of thephysical variable, or ymax and ymin.

According to known principles of grid responsive loads, at a particularsetting of the load, the input energy is varied to keep the physicalvariable y within the limits set by ymin and ymax, in the same way asthe load would normally be operated, except the frequency of the grid(or some other parameter associated with the balance between generationand load on the grid) is taken into account.

Speaking generally, a load, of the type applicable with the presentinvention, operated without a grid responsive controller would switchthe load on when the minimum value of y (ymin) is reached and switch theload off when its maximum value (ymax) is reached.

According to the grid responsive controller of the preferred embodiment,the timing of the switching, when an on load is switched off or when anon load is switched on, is adjusted depending upon the frequency of thegrid. During a period of low frequency, for example, there is too muchload on the grid and not enough generation to match it, and a gridresponsive device which is on will react by switching off (or switchingto a decreased energy consumption state) before it would normally haveswitched off, i.e. before y reaches ymax. Likewise, during a period ofhigh frequency, more load is needed to take up the excess in generationand the loads will be switched on (or switched to an increased energyconsumption state) before ymin is reached.

Further, an extended set of upper and lower limits for the sensedvariable can be determined in order to improve the amount of responseprovided. So, during a period of high frequency, grid responsive loadswill be switched on and the maximum value for the sensed variable (ymax)can be increased such that the loads having been switched on remain onfor a longer than normal period of time, as will loads that were alreadyon. A similar provision is utilized during periods of low frequency.

The preferred control device of the present invention defines a statusfor the grid, so as to determine the exact type of response to changesin frequency provided by the grid responsive control device. The gridresponsive control device has three modes of operation, a “normal” mode,a “stress” mode and a “crisis” mode, in a similar way to the systemdescribed in UK patent application no. 0322278.3.

The preferred embodiment of the present invention determines the mode ofoperation of the controller and the associated grid status from adefined function of the frequency, hereinafter called h. The function hdetermines from the behavior of the grid frequency the current status ofthe grid. Ideally h represents to some extent a measure of how muchenergy has been borrowed from or loaned to the energy store loads.

The function h preferably includes three principal terms, aproportionate term, an integral term and a derivative term. These threeterms will give a good indication of the stability state of the grid.

The proportionate term is the current frequency deviation from thenominal frequency of the grid or some other central value thatrepresents how much the frequency needs to be corrected to return to thedesired central value.

The integral term represents a longer term (as compared to theinstantaneous proportionate term) view of the frequency error. This termis useful, as a small error for a long time, will influence the functionh and, thus, be taken into account in providing grid stability response.The integral term can be a sum of a set amount of past frequencydeviations or can be a moving average of past frequency deviations.Rather than from time zero, the integral term can be measured since thelast time the frequency deviation was zero.

The derivative term is related to the current instability of the grid.It can be a rate of change of frequency deviation. Thus, large swings infrequency will also affect the function h and can be an indication of anunstable grid, even if the actual present deviation of the gridfrequency is not outside preferred limits.

In equation form, the function h can be written

h=Pfc+ICc−Df′c

where fc is the proportionate term, Cc is the integral term and f′c isthe derivative term. P, I and D are constants for influencing the degreeof importance to the function h of each of the terms.

The integral term Cc may be calculated by (fcS), where S is the sampleintegral.

The three parameters, P, I and D should be enough for the control deviceto derive h, but for completeness and flexibility, it may be appropriateto extend this to quadratic or cubic terms.

According to the preferred implementation of the present invention, thegrid status is inferred from the function h. For example, if h is belowa first limit, then a “normal” status of the grid is determined. If h isbetween the first limit and a second, greater limit, than a “stress”condition for the grid is determined. If h is between the second limitand a third, greater, limit, then a “crisis” condition is determined.The difference between the modes of operation associated with each ofthese grid states is similar to that described in UK patent applicationno. 0322278.3.

The function h is a useful way of determining the stress under which thegrid is operating. Appropriate setting of the parameters P, I and D of henable the function to appropriately distinguish the three generalstates of the grid.

During the normal mode of operation, the grid responsive control deviceof the present invention will operate as fully described below. Duringstress mode of operation, a user of the energy store load is not allowedto adjust a setpoint of the physical variable associated with the load.Thus, negation of the grid responsive compensation provided by thepresent invention is not possible. During a crisis state, the energystore load is operating without regard to the desired range of thephysical variable associated with the load. The load's physical variableis allowed to reach the absolute limits of y rather than the preferredrange represented by ymax and ymin. For example, in a crisis state, arefrigerator could be allowed to reach an ambient temperature, or beallowed to go to the lowest temperature that the refrigerator is capableof achieving. Similarly, in the case of a water tank, the water levelcould be allowed to reach empty or extend up to a full tank level.

A main mode of practicing the principles of the present invention is nowdescribed. Other preferred embodiments of the invention follow.

The grid responsive controller of the present invention includes acontrol mechanism for actively and continuously damping grid frequencyvariation. The grid responsive control device of the present inventionis responsive to all frequency variations from a central value, which isdefined as an average value over a predetermined sample period ofhistorical frequency readings.

When the control device is first used, the central value will be set tothe current frequency. The central value will then evolve as pastsamples of the grid frequency are incorporated into the moving average.The central frequency is the average value of the grid frequency sincethe start of the sample period.

Any movement of the grid frequency from the central frequency isresisted by the population of responsive control devices of the presentinvention. If the current frequency is above the central value, then theresponsive control devices will tend to switch on their loads tocompensate for the increase. If the current grid frequency falls belowthe central value, then on devices will tend to switch off to compensatefor the deficit in generation. This provides an overriding stabilizingeffect on the grid, as represented by a clearer, or less noisy, gridfrequency signal.

The loads will not all change energy consumption status at the sametime. The control device of the present invention is adapted to ensurethe loads are switched in a progressive way such that greater deviationfrom the central value results in more loads tending to switch on/off.This progressive switching is important in order to ensure that theresponse of a population of loads is not simultaneous, which wouldprovide a destabilizing influence to the grid. The randomization isdescribed in more detail below.

In the preferred implementation of the responsive control device of thepresent invention, the sample period of calculating the centralfrequency value is taken as the period since the central frequency lastcrossed the nominal frequency of the grid.

The present invention defines high frequency excursions, when thecentral frequency moves above nominal, and low frequency excursions,when the central frequency is below nominal. The end of one of thesetypes of excursions marks the start of the other. These cross-overs havebeen found to be a convenient time for beginning accumulation offrequency readings for calculation of the central frequency. Thus, thecentral frequency will be calculated for each high excursion (abovenominal) or low excursion (below nominal) of the central frequency. Thecentral frequency will, therefore, be calculated as a moving average ofthe frequency during the current excursion and is reset once the centralfrequency crosses nominal and a change of excursion (e.g. high to lowexcursion or vice versa) occurs.

An advantage of choosing above nominal or below nominal excursions forthe sample period is that devices will end-up having a shared commonview of the central frequency. Loads that are recently connected to thegrid, and so have no history, will soon come to see the same recenthistory as other devices, since the central frequency crossing over thenominal frequency of the grid is expected to occur frequently enough. Itis useful for the devices to appreciate a common central frequency as itallows their behavior to be coordinated (but not synchronized) in anintended manner.

This sample period may not always be appropriate. If the excursion lastsfor a period that approaches the average on or off cycle of the energystorage device, the devices may well be called upon to provide gridresponsive behavior without having had the opportunity to reach theirmaximum or minimum energy store. This could have an adverse effect onswitching rates of the energy storage loads. Further, if the load doesnot reach its maximum energy store, and fully replenish itself, then thepopulation of such loads will, on average, be depleted. It may be thatthe control device will have to be adapted slightly in order to beuseful in such circumstances.

It is envisaged that the moving average for obtaining the centralfrequency could be a weighted moving average, such that the most recentfrequency terms are given more importance. In this way, movements offrequency from recently obtained values will more likely provide loadresponse and could be compensated for. This will further help tostabilize any frequency movement of the grid.

The grid responsive control device of certain aspects of the presentinvention also includes a further improvement aimed to minimizeswitching of a load and to distribute energy variations across theavailable population of the loads. As described in further detail below,this is achieved by varying the trigger frequency of the device as itprogresses through the current on or off state.

A trigger frequency is the frequency of the grid at which the load willbe controlled to switch from an on state to an off state or an off stateto an on state. The loads will also be switched on or off when thesensed variable associated with the load reaches its current minimum ormaximum, as defined by ymin and ymax.

The grid responsive control device is adapted to determine a target (orbase) frequency in a random way. In a population of such devices, thetarget frequencies will be distributed randomly across the population sothat the above described progressive response is achieved.

According to a preferred embodiment of the present invention, thedevice's target frequency is the frequency to which, on average, thedevice will respond. The current triggering frequency, however, which isthe frequency of the grid at which the load will switch between on andoff states, is not usually the same as the target frequency. The targetfrequency is a randomly chosen frequency, from which a unique profilefor determining the trigger frequency, the grid frequency that willcause the device to trigger between states, is derived.

So, the profile for the trigger frequency is derived from a function,which, in turn, is dependent upon the randomly chosen target frequency.The actual trigger frequency used by the control device is derived fromthis function, which is preferably a function of how long the device hasbeen in its current energy consumption state, i.e. how long it has beenon for or off for.

How long a device has been on or off for, is determined relative to anatural switching point, which is the point at which the sensed physicalvariable will reach its current maximum or minimum values for the sensedphysical variable (ymax and ymin) and would, therefore, switch anyway.Thus, the function for determining the device's trigger frequency isalso a function of the value of the sensed variable relative to itsminimum or maximum values.

The current trigger frequency is, therefore, dependent upon the currentvalue of y. According to the preferred embodiment of the presentinvention, the trajectory of the trigger frequency is biased such thatthe further away a load is from its natural switching point, the triggerfrequency will be a less likely frequency of the grid, i.e. the triggerfrequency will be further away from nominal. Thus, the device is lesslikely to switch the further away it is from a natural switching point.

Preferably, the trajectory of the trigger frequency is biased in such away that half the time the device is less sensitive than the randomlychosen target frequency, and half the time it is more sensitive. Thus,preferably, the average of the trigger frequency is the targetfrequency.

In the preferred embodiments, the length of time in which a load hasbeen in either the on or the off state is calculated from the currentvalue of the sensed variable as compared to a range allowed for thatvariable as defined by the current values of ymax and ymin. This could,for example, be expressed as a percentage. For the sake of illustration,a load device which is in an on state with the sensed variable close toreaching a maximum of the sensed variable could have been on for 80% ofits normal on period. This can be expressed formulaically as

ton=(y−ymin)/(ymax−ymin)

where ton is the amount of time that the load has been switched onrelative to its expected on time and y is the current value of thesensed variable.

How long the device has been off for is defined using a differentformula, but the same principle applies. The closer an off device is toits lower limit ymin, the longer it has been off for. Thus, theappropriate formula is as follows:

toff=(ymax−y)/(ymax−ymin)

where toff is the relative amount of off time as compared to theexpected amount of off time for the load.

FIGS. 1A, 1B and 1C show example forms of the profile of the triggerfrequency function. Frequency is plotted up the y-axis and percentfullness/emptiness, in terms of energy, of the energy store load isplotted along the x-axis.

FIG. 1A shows the frequency at which an on device will switch off. As isunique to the present invention, the trigger frequency is dependent uponthe time for which the device has been on, as compared to the expectedtime (ymax reached). As can be seen from FIG. 1A, for 50% of the time,the trigger frequency is relatively close to the central or nominalfrequency, for the other 50% of the time, the trigger frequency isfurther away from these frequencies. Thus, it is only during the moreextreme grid circumstances that the devices which have only been on for50% or less of their expected on time will be triggered. This is basedon the assumption that the grid frequency will, for the majority of thetime, reside around the central or nominal frequency and, thus, triggerfrequencies that are closer to this are more likely to be achieved bythe grid. Thus, switching the load is more likely to take place thecloser the triggering frequencies are to the nominal or centralfrequency.

It is also important to appreciate that the exact form of the triggerfrequency's dependency upon time on or off compared to the expected timeon or off is chosen by the target frequency, which is randomly chosen.In this way, a population of loads will provide a diversified gridfrequency response.

Comparing FIG. 1B with FIG. 1A illustrates the profile dependence uponthe target frequency chosen. It can be concluded that while the triggerfrequency is always varied with the percent of expected on or off timeof the load, the form of this variance is determined by the randomlychosen target frequency.

FIGS. 1A and 1B show the trigger frequency for an on device. FIG. 1C,conversely, shows the profile for an off device. The principles are thesame. Namely, the frequency at which an off device will switch on variesdepending upon the percent of expected off time, as defined by the aboveformula. As can be seen from FIG. 1C, the trigger frequency approachesthe central frequency or the nominal frequency of the grid as the deviceapproaches its natural switching on point. Generally, the profilerequires that the closer the device is to its natural switching onpoint, the closer the frequency to the damping or nominal frequency and,therefore, the more likely the load will be used for providing gridfrequency response.

According to the preferred implementation of the invention, any movementof sensed grid frequency above or below the central frequency willresult in loads being switched. The further the sensed grid frequency isfrom the central frequency, the progressively more loads that willswitch. Since the central frequency is a moving average of pastfrequency ranges, the central frequency will tend to “follow” the sensedgrid frequency, although in a damped manner. This will provide a smoothcentral value for using to determine whether to perform high frequency(above nominal) or low frequency (below nominal) response.

The sensed frequency may well change direction and go above or below thecentral frequency. The device of the present invention will resist anyrapid increases or decreases in the grid frequency above or below thecentral frequency by borrowing or repaying energy from or to the grid assoon as the grid frequency starts to move. This is the appropriate timefor the energy borrowing or repayment, as discovered by the presentinventor, and provides a far more stable grid frequency, as compared tothe prior art grid responsive control frequencies.

At first, any movement above or below the grid central frequency onlyswitches loads that are near to their natural switching points. This isbecause of the trigger frequency being variable for a particular devicewith on or off time for a device. All loads that have been on or off forgreater than 50% of their expected on or off time are favored. It isonly when the grid frequency moves dramatically away from the centralfrequency that devices that are less than 50% of the time away fromtheir previous switching point will switch.

Thus, the preferred implementation of the present invention provides amore stable grid frequency, thereby inherently resulting in lessswitching of the responsive load. Furthermore, switching of devices thathave already been switched is disfavored, thereby further decreasing theswitching burden on the load.

A system consisting of a population of energy store loads controlled bythe grid responsive control devices of the present invention provides apopulation of loads ready to switch in response to any change in thegrid frequency. The larger the change in frequency, the larger thepopulation of loads providing response. This should be a linearrelationship.

FIG. 2 shows an example of a system being controlled in accordance withthe present invention, in a stable state and running at the grid'snormal frequency. As shown in FIG. 2, in this state the portion ofdevices that are off [1] and the portion of devices that are on [2]corresponds to the expected duty cycle. So, if the load is run at a 50%duty cycle, the population is evenly divided.

If the system moves into a low (below nominal) frequency excursion, onloads will be triggered off [3] in order to reduce the load. They willbecome unlikely to switch on again for a while.

During this low frequency excursion, some off loads will be switched on[4], despite the current excess of load on the frequency, because of thefact that they have reached their minimum energy store state and properfunction of the load requires it to switch on. These loads were notcalled on to provide high frequency response, and are lost from thepopulation of loads capable of providing high frequency response, eventhough they were actually the most sensitive. Again, these recentlyswitched loads are unlikely to switch on again for a while.

Some loads will reach their maximum energy state, and will need to beswitched off [5]. If the duty cycle is 50% the number of devicesreaching their maximum energy state [5] will tend to be the same as thenumber of devices reaching their minimum energy state [4].

The remaining devices capable of providing low frequency response [7] isthe population that had the less sensitive frequency settings, sincethose close to the nominal grid frequency have been “used up”.

If the frequency now rises above the central frequency, then, despitethe frequency still being below the nominal grid frequency, it isdesired that the loads begin to switch on and start recovering theenergy loaned to the grid earlier.

As the frequency rises above the central frequency, some devices will betriggered on [8] in order to increase their load. These are most likelyto be drawn from the population remaining from [1], as the loads [3]will be in a minimum switching mode since they were only recentlyswitched.

As before, some on devices will come off [10], and some off devices willcome on [9] because they have reached their maximum or minimum energystate, respectively, without being called upon to provide high frequencyresponse. While the on devices coming off [10] were the most sensitivefor providing response of the population, they were lost to thepopulation for providing low frequency response, without being used.This population of loads reaching their minimum or maximum energy stateswill be quite small.

The population of devices continuing to be able to provide highfrequency response is as desired, in as much as they are reasonablyevenly distributed amongst the frequency between the central frequencyand a maximum limit frequency. The population of devices sensitive tofrequencies immediately below the central frequency has, however, becomedepleted. So a downturn in frequency again will trigger less loadreduction than before resulting in an average frequency that will falleven further until an undepleted zone is reached, or the naturalmigration of trigger frequencies repopulate the depleted zone.

This is desired behavior. During a low frequency excursion where thefrequency is undulating, the frequency will tend to fall more easilythan it rises (or, more generally, move further from nominal more easilythan it approaches nominal). This reflects the fact that the loads arelending energy to the grid and resisting rising frequency as the loan isrepaid. Ideally, it is only when the loan is fully repaid that thefrequency returns to nominal.

One possible manipulation of the low frequency population shown in FIG.2 is to distribute remaining on loads across the range between thecentral frequency and the minimum frequency (rather than between thenominal frequency and the minimum frequency). This has the effect ofleaving the frequencies immediately below the central frequencyadditionally depleted (as devices that would have chosen targetfrequencies above the central frequency, between the nominal frequencyand the central frequency now have them below the central frequency) sothe frequency has a greater tendency to drop. This may not be desirable.

In an alternative to this manipulation, this change could be made onlyfor some loads, such as those that have switched on since the start ofthe low frequency excursion. The logic of this arises because thedevices which have switched on since the start of the excursion wouldhave done so because they are low in energy, and so need an opportunityto replenish this energy before they provide a response. One way toaffect this is to systematically lower (make more extreme) the frequencyat which they will switch on. This, in turn, will tend to allow the gridfrequency to fall further. It will, in extreme circumstances, also tendto distribute the on time more evenly among devices. The on time is,without this modification, already evenly distributed across devices bythe trajectory of the trigger frequency.

The example shown in FIG. 2 is for a low frequency excursion, thebehavior of a population of loads during a high frequency excursion issymmetrical.

In an ideal system, where all grid response is provided by devicescontrolled according to the present invention, frequency excursions willnot end until energy borrowings have been repaid. If the response togrid frequencies comes from other sources as well, (i.e. by generators),the excursion may end before borrowings have been fully repaid, but theloads will nonetheless retrieve the energy required to replenish theirenergy store.

The central frequency, derived from a moving average of the frequency isthe frequency above which the overall load derived from devicescontrolled according to the present invention will increase, and belowwhich the load will decrease. This is, effectively, a target frequencyfor the whole system. It could be that a system target frequencydifferent from this could be further derived. The possibility is to movethe system target frequency closer to the nominal frequency, so as toprovide some bias to influence the grid frequency towards nominal.

Below is described, in further detail, a complete procedure forobtaining the triggering frequency for a particular control device.

First, the central frequency is calculated. Each reading from the firstrecorded frequency reading since the current excursion above or belownominal began is taken into account. The obtained central frequency maythen be further manipulated to bias it towards the nominal frequency,but this may not be necessary since such a bias is an inherent featureof the control devices of the present invention.

A device base or target frequency is then determined. To do this, arange within which the base frequency is to be placed is determined andthen a random target frequency is chosen within this range. Each devicehas both a high target frequency and a low target frequency, which arepreferably provided from separate random values called the low randomvalue and the high random value. The high target frequency is for a highfrequency excursion and the low target frequency is for a low frequencyexcursion.

When choosing the random number which distributes the target frequencybetween the nominal and low limit or high limit of the permissiblefrequency range, it is preferred that one random number is used for highfrequency excursions, and another random number is used for lowfrequency excursions. The random numbers are preferably provided between0 and 1 so that the target frequency can be positioned anywhere betweenthe full range (as defined above) of possible frequencies. It ispreferred that the two random numbers are regenerated after an oppositeexcursion begins.

Thus, the low frequency excursion random number is chosen at the startof the high frequency excursion and the high frequency excursion randomnumber is chosen at the start of a low frequency excursion, therebyensuring the readiness of the random number upon a frequency excursionchangeover.

It is important to regenerate the random numbers at regular intervals,as the sensitivity to grid frequency changes for a particular controldevice depending upon the random number. As will become clearer below, arefrigerator with small random numbers will tend to carry a greaterswitching burden than one with larger random numbers. This is becausethe target frequency generated from a large random number will be morelikely to provide a triggering frequency closer to the outer frequencylimits, which are more rarely realized by the grid, than frequenciescloser to the nominal frequency of the grid.

It is also important that the random number is not regenerated during aparticular excursion. This could result in an unpredictable impact uponthe grid stability. Other strategies are possible, however. For example,the random numbers may be generated during a first change following a 24hour period or other such chosen period.

There are four possible ranges within which the target frequenciesshould be provided:

(1) The grid is in a low frequency excursion (central frequency belownominal) condition and the load is currently on. This is shown in theleft hand side of FIG. 3A. In this case, the target frequency isprovided between a low frequency limit (the selection of the lowfrequency limit for the grid is discussed below) of the grid and thenominal grid frequency. Since the grid is currently in a low frequencyexcursion, the central frequency will also be provided between thenominal frequency and the above low frequency limit.

(2) In the case of a low frequency excursion when the load is off (FIG.3A right hand side), the target frequency will be randomly positionedbetween a high frequency limit (the selection of the high frequencylimit for the grid is discussed below) of the grid and the centralfrequency (different from the nominal frequency).

(3) In the case of a high frequency excursion (central frequency abovenominal) and the load is off (FIG. 3B left), the target frequency israndomly provided between the high frequency limit and the nominal gridfrequency.

(4) In the case of a high frequency excursion and the load is on (FIG.3B right), the target frequency is provided between the low frequencyand the central frequency value.

FIGS. 3A to 3B show example positions of the frequencies in each ofthese four possibilities. These figures also show the triggerfrequencies, at which point the grid frequency will be such that ittriggers a particular device off if it was already on or on if it wasalready off. FIGS. 3A to 3B show that the triggering frequencies areprovided within the same range of frequencies provided for the randomplacement of the target frequency.

As shown in FIGS. 3A to 3B, the device target frequency is determinativeof the form of the triggering frequency profile. Thus, the randomizationof the target frequency is carried through to the triggeringfrequencies.

In the preferred implementation of the control device of the presentinvention, once the high or low target frequency has been calculated fora particular device, the devices specific trigger frequency needs to becalculated. When the device is on, only the low target frequency isrelevant and when the device is off only the high target frequency isrelevant. From the value of the particular target frequency, the form ofthe function can be derived. The function is different, not onlydepending on the target frequency for the device, but also on whetherthe device is on or off. From this function, using the current value ofthe sensed variable, the device's current triggering frequency can beobtained. This triggering frequency is then determinative of whether thedevice will switch on or off by comparing it to the sensed frequency.

The value of the triggering frequency shown in FIGS. 3A to 3B iscalculated as outlined below. The proportion referred to below is arepresentation of how close the energy store is to being at its maximumor minimum depending on whether the device is on or off, respectively.The proportion is preferably ton or toff, the calculation of which isdescribed above.

(1) If the proportion is less than 0.5, then

(i.e. is the time since the load switched last less than 50% of the timeit takes to reach minimum or maximum)

(2) “Offset”=(the target frequency−“StartPoint”)*the proportion

where the StartPoint is the low frequency limit for on devices and thehigh frequency limit for off devices. Thus, the (targetfrequency−StartPoint) is the difference between the high frequency limitor the low frequency limit and the target frequency. Since theproportion always runs between 0 and 0.5 (as per step (1)), thisdifference is made smaller by the proportion term. Thus, in this step,the value of the sensed variable is influencing the triggering frequencyas is the target frequency.

(3) the trigger frequency=Startpoint+OffSet

thus, for on devices the trigger frequency is offset from low frequencylimit and for off devices, the trigger frequency is offset from the highfrequency limit.

(4) If the proportion is greater than or equal to 0.5, then

(i.e. is the time the device has been on or off more than half waytowards its natural switching point? If so, then the load needs tooperate in a higher probability switching zone than above).

(5) Offset=(“Endpoint”−the target frequency)*the proportion

where the Endpoint is the central frequency for off devices during lowexcursions and for on devices during high excursions and is the nominalfrequency for on devices during high excursions and off devices duringlow excursions. The offset is the difference between the targetfrequency and the endpoint, with the difference factored by theproportion. Since the proportion is always between 0.5 and 1, the offsetis somewhere between being all or half of this difference. Again, thisstep shows that how long the device has been on or off and the targetfrequency both influence the value of the offset.

(6) the trigger frequency=the target frequency+Offset

thus, the trigger frequencies are provided between the target frequencyand the central or the nominal frequency.

A load control device having the triggering profiles shown in FIG. 3Awill now be described.

During a low frequency excursion, the central frequency is providedbetween the nominal and the low limit for the grid frequency, as shownin FIG. 3A. During such a low frequency excursion, the overall desiredbehavior is for on devices to tend to switch off in order to eventuallybring the system frequency back towards nominal.

FIG. 3A shows the evolution of a load which is initially in an on statewhile the grid is in a low frequency excursion. The trajectory of theenergy state 1 (left hand axis) shows it moving from a minimum energystate towards a maximum energy state. If no response is provided, theload will switch off at the maximum energy state from its limit setting,and the energy state will then move from the maximum to the minimum.

For each reading of the grid frequency and the physical variableassociated with the load, the central frequency is recalculated. Forclarity, the diagram shows a fixed central frequency, but it willactually vary with grid conditions.

While the device is on, the target frequency for off is then calculatedusing the low frequency random number 2. This will lie over the range 3shown on the left of the diagram (FIG. 3A), which, in this state, ischosen to be between the low frequency limit and the nominal frequency.The physical variable associated with the load is then used to calculatethe trigger frequency for off 4. The triggering frequency for on devicesthus takes into account the new central frequency and the new sensedvariable associated with the load. For on devices, when the gridfrequency is below the target off frequency, then the load will beswitched off. For off loads, when the measured grid frequency is greaterthan the triggered on frequency, then the load will switch on.

When the grid frequency becomes lower than the trigger frequency 5 thedevice will switch off, and the energy trajectory will change direction,even though the maximum energy store has not been reached. This has theeffect of lowering the average energy stored in the device withoutchanging the physical variable limits. In a large population of devices,this has the effect of raising the average temperature of the populationof devices.

When the device has switched off, its further behavior is shown on theright hand side of FIG. 3A. The portion of the duty cycle that is missed6 is shown hatched.

In this case the range over which the target on frequency is chosen liesbetween high frequency limit and the central frequency, and an exampletrajectory 8 of the trigger frequency for on is shown. If the centralfrequency stays unchanged, then the device will not switch on again 9until the energy state has once again reached its minimum.

According to FIG. 3A, any movement of the measured grid frequency awayfrom the nominal will result in loads being switched off. Clearly, thefurther the grid frequency is from nominal, the progressively greaternumber of devices that are switched off. Also, it can be seen that thefurther the sensed frequency is from nominal, the earlier the load willtend to be switched off during its on cycle.

According to FIG. 3A, any movement above the central frequency will tendto result in the off loads being switched on. Thus, the triggeringfrequencies provided by the present invention resist all grid frequencymovements about the central frequency.

A similar discussion is applicable for high frequency excursions, asshown in FIG. 3B.

In a manipulation of the shown embodiments, all four of the ranges forprovision of the target frequency, described above, could be providedbetween the central frequency and a maximum or minimum limit, ratherthan two of the ranges being between the grid nominal frequency and ahigh or low limit (as in FIGS. 3A to 3B). In this alternative form ofthe control device, FIG. 3A will be adjusted such that only decreases inthe sensed frequency below the central frequency will result in loadsbeing switched off (rather than decreases below central and increasesabove central up to the nominal frequency, as is shown). This will stillprovide the desired response, as a reduction of frequency means too muchload and, therefore, devices switching off. Similarly, the profile ofFIG. 3B could be modified so only increases above the central frequencywill result in off devices coming on (rather than increases abovecentral and decreases below central up to the nominal value, as isshown). Again, the response provided in this modified form is still asdesired since a rise in frequency represents an increase in generation,which needs to be taken up by switching loads on.

According to FIG. 3A, during a low frequency excursion, if the currentsystem frequency falls below a central frequency, then the off devicescannot switch on, since no triggering frequencies are provided belowthis point. The only way in which off devices will be switched on wouldbe if the physical variable of a load reaches its lower limit. Thus, inthe case of a decrease below the central frequency, response is onlyprovided for on devices to switch off, as can be determined from FIG.3A, which is exactly as required to compensate for the excess loadcausing the frequency drop.

Again with reference to FIG. 3B, during a rise in frequency above thecentral value, it is desired that off devices begin to switch on. Thisbehavior is provided according to FIG. 3B. FIG. 3B also shows howdevices approaching natural switching on points are favored by havingtheir triggering frequencies closest to the central frequency. Thefigure also shows how the triggering frequencies of the population ofthe off devices are spread between the central frequency and the highfrequency limit so as to provide progressive response behavior.

There is the possibility that the frequency of the grid will repeatedlymove up and down within a narrow frequency range close to the centralfrequency. In these circumstances the population of devices sensitive tothe experienced frequencies will become depleted. That is, when thefrequency falls, the most sensitive devices will switch off, and when itrises the most sensitive devices will switch on. The devices that switchin this way will become unavailable for providing further response untilthey have completed the remainder of their cycle. In due course, thepopulation of sensitive devices will be restored as devices approach thestate in their cycle, which may be shorted by the response it provides,where they are again willing to switch.

The rate at which a depleted frequency zone is replenished is influencedby the range over which the target frequency is chosen. Including thefrequency zone that is depleted of sensitive devices into the targetfrequency range, increases the rate at which depleted zone isreplenished from the population of devices that are approachingsensitive points.

The increased replenishment is achieved by spreading the depletionacross a wider frequency range, which is not currently being experiencedon the grid. While this does reduce the total response still available,this properly reflects the physical fact of using up the finite responseprovided by the population of fridges.

Although it appears that there is a zone between the Nominal frequencyand the central frequency where the action of on devices and the actionof off devices appear to overlap and so negate each other, in practice,these actions will not in fact take place at the same time, but areseparated by the time taken for the grid frequency to change directionand serve to damp small and up and down frequency changes.

The degree of Response available as the grid frequency passes through adepleted zone will be less, so the change in load available to slow thechange in frequency will tend to be less. This tendency makes thefrequency a more accurate indicator of the extent to which energy hasbeen loaded to or borrowed by the fridge population and is the intendeddesired behavior.

As can be seen from FIG. 3A, when the sensed grid frequency increasesabove the central frequency, off devices come on according to FIG. 3A.Only if the frequency then again decreases will on devices turn off asthere remains a population of devices with the trigger off frequenciesbetween the central frequency and the nominal frequency. This means thatwhile movements of the grid frequency below the central frequency willresult in only on devices being switched off (excluding off devicesreaching their minimum limits of the physical variable associated withthe load), the response for the grid frequency moving above the centralfrequency is provided by off devices switching on, as desired tostabilize the grid frequency movements.

A similar discussion to the one given above concerning low frequencyexcursions with regard to FIG. 3B, is symmetrically applicable to a highfrequency excursion (above nominal) of the central frequency.

In a real grid, changing between high and low excursions as the load andgeneration varies, the population of fridges in each state will bedynamic and the behavior of individuals fridges less determined than inthese descriptions.

The maximum and minimum frequency limits are used by the control devicefor determining the ranges over which the target and triggeringfrequencies should be spread. These frequency limits can be determinedby experience over time of the frequency behavior of the grid or can beset at installation depending upon the grid with which it is intended tobe used.

For example, in the US, the grid frequency is intended to be kept withinplus and minus 0.5% of the nominal grid frequency, i.e. the gridfrequency should always fall between 59.7 hertz and 60.3 hertz. Thiswould be the default value for a control device intended to be operatedon the US grid. These default values could be set or could beself-optimizing based on the device's experience with the grid. Thepossibility of a self-optimizing control device for providing thesefrequency limits will now be discussed.

The control device of the present invention will be preferably providedwith a default set of parameters related to the grid with which it isexpected to be used. As can be seen from FIGS. 3A to 3B, if the gridfrequency passes outside the maximum or minimum range, the entirepopulation of devices will be in the same switched state, i.e. eitheroff or on. No further grid response is available from the load. Thus, itis important to perform a self-tuning of the frequency limits correctlyand carefully.

Ideally, the frequency control limits are chosen to lie just beyond thefrequency deviation tolerable by the grid. It is also, however,desirable to keep the rate at which the grid frequency varies fairlylow. The method adopted by the grid responsive control device of thepresent invention is to balance these requirements to monitor thefrequency extremes experienced, and to use these to adjust the frequencylimits stored. Two core adjustment processes are used.

First, if the extreme frequency experienced during an excursion isgreater than the limit used, then, in subsequent excursions, the extremewill become the new limit. So on a grid with big variations, the gridresponsive control device will adjust to distribute its service acrossthe full range of frequencies experienced. The grid responsive controldevice has the capacity to analyze the events leading up to the extreme,and can use this to moderate the extent to which the limits are widened.

In the second process, if the extreme frequency experienced within aperiod is less than the currently stored frequency limits then thefrequency limits will be adjusted to be closer to the frequency extremesexperienced. The responsive control device will, however, only bring thelimits closer by a small proportion of the difference between theextremes and the limits (a moving average technique). In this way, itwill take numerous cycles of adjustment before the frequency limitsbecome significantly narrower. The tendency for the limits to narrowcould also be countered by ignoring all excursions outside the storedfrequency limits that are shorter than a defined period (for example inminutes).

So, if the device experiences more extreme frequencies than its defaultslead it to expect, it will rapidly widen its behavior to suit thecircumstances. If, on the other hand, the grid is more stable than thedefaults lead it to expect, it will only slowly migrate towards narrowerlimits, and will still react quickly if the grid behavior again becomesmore volatile.

Further, the limits are provided with a margin, a so-called rare eventmargin, such that the grid responsive control device will assume thatthe biggest frequency excursion is not rare, and so the frequency limitsactually chosen are adjusted to provide spare capacity proportionate tothe rare event margin. The rare event margin could be provided, atmanufacture, in two ways.

The rare event margin could be set to be less than unity meaning thatgrid response behavior will not be possible whilst normal extremes ofthe network are experienced. This is because the rare event margin willdefine the control device's frequency limits to lie within the grid'sfrequency extremes. In a grid where grid responsive behavior ispredominantly provided by fossil fuel plants and not by the loads,substantial emissions benefits can be achieved with a rare event marginof less than unity.

Alternatively, the rare event margin may also be set to greater thanunity. Thus, the grid responsive control device will tune itself so thateven during grid extremes, there is a margin for exceptional events.This mode is essential when the grid responsive control devices of thepresent invention are the predominant provider of grid responsivebehavior, as some responsive behavior in all grid circumstances will beneeded.

Thus, the rare event margin of less than one will be used at the earlystages of implementation of the grid responsive control device and asthe population grows, a rare event margin of greater than unity willbecome the normal standard.

The emissions benefit of having a rare event margin of less than onearises because providing response at the load end will not have anyimpact on emissions, whether of carbon dioxide or other pollutants. Thisis in contrast to providing response at the electricity supply end,where the generation plant will have to be operated at less thancapacity and be able to operate with frequent dynamic changes (makingefficiency and pollution control harder).

In order to conclude when an extreme frequency or rare event hasoccurred, the grid responsive control device of the present inventionneeds some definition of “rare” to use. Extreme grid events include afailed generation plant or a failed important transmission line. Such anevent is most unlikely to happen any more than extremely infrequentlyand it is the sort of event that the rare event margin of greater thanone is intended to cover. On the other hand, if a transient peak loadoccurs, such as a TV break in winter, is not covered by the extremefrequency limits, then the limits should usefully be adjusted to coversuch an event, which is an indication of grid stress, but not a rarefailure.

It may also be worth considering having different frequency controllimits for different various periods within a day or a week (many gridsuse half hours as metering boundaries, and this may be useful here). Therange of the limits may be wider at times when the demand is changingrapidly, as indicated by the stress status function (h) defined earlier.Minimum demand times or a low stress state of the grid could have anarrowed range of frequency control limits. The times during a day whenthe grid is most likely to be stressed could be learned from experiencewith the grid and the intervals at which the frequency control limitsneed to be widened could be timed by the control device. Since, however,the control device will not have access to an external clock, thistuning will need to be discarded whenever the power is switched off.

In overview, the present invention provides a grid frequency responsecontrol device that minimizes switching of loads, resists all changes offrequency about a historical moving average of the current frequency andbiases the system frequency towards nominal to some extent. Thus, thegrid is stabilized and overworking of the loads is prevented. A clearfrequency signal is also provided that is less noisy, is smoother andwhich is gradually and continuously biased towards an ideal nominalfrequency of the grid.

In the above, switching the energy consumption of the load between onand off states is performed by directly controlling the energy consumingdevice of the load. However, an alternative implementation of thepresent invention is to adjust the set point or the central limits ofthe parameter of the load. In this way, the load will adjust its energyconsumption to keep the sensed variable of the load within the controllimits.

In the example of a refrigerator, when the frequency sensed is such thatthe refrigerator should switch on, the control limits can be shiftedbelow the present value of the temperature of the refrigerator's coolingspace. This, the control mechanism of the refrigerator will detect thatthe temperature is too high and respond by switching the cooling meansof the refrigerator into an on state. The opposite direction of movingthe control limits can be performed when the frequency is sensed asbeing such that the refrigerator should switch off.

Instead of adjusting the control limits, the set point itself can beadjusted by the control device of the present invention. The controlmechanism of the load will receive the new set point and derive thecontrol limits itself.

The control device controlling the setpoint or the control limits inthis way may be advantageous. Such a control device will not need to beintegrated into the control circuitry of the load so as to be able todirectly communicate with the energy consuming means of the load.Instead, it merely needs to provide a central signal to the controlcircuitry of the load and the varying of the energy consumption isperformed in the normal way.

We have up to yet discussed preferred embodiments where grid responsivecontrol is performed by switching the energy consumption either on oroff. Some loads, however, control a physical variable of the load withincontrol limits by adjusting the level of energy consumption. Thus, theload may be controlled between a first state of increasing the energystored by the load and a second state of decreasing the energy stored bythe load, as has previously been discussed. Below is described anexample implementation of the control device of the present inventionwith a refrigerator using such continuous control of the energyconsumption to maintain the temperature of the cooled space withincontrol limits.

A pure temperature controller will likely aim to become a classic threeterm controller, with parameters influencing the extent to whichvariations from the set point influence the power. Classically, theseare Proportionate error (how big is the error now); the Integral error(accumulating smaller error over time), and the Derivative error (sothat, if the error is reducing rapidly its overshoot is minimized). Thisis known as the physical PID controller, although the control may, infact, not include all three terms and so be simpler than this.

In general, the PID controller actually drives a motor power controller,which in turn drives the power electronics of the motor controller thatactually drives the motor or load. FIG. 6 gives further detail:

The Manual Controller provides input to a Set Point Controller thatprovides the set point signal to the PID controller in a suitable form.The PID controller also has as input the current state of the variablebeing controlled, so, in a fridge, for example, this would be thetemperature.

The output from the PID controller is a desired motor power level. Thisis the power level considered appropriate to keep the controlledvariable at its set point.

This desired power level is often used by a further controller to makeadjustments to the actual power flowing to the motor, as the rate ofchange of the actual power may be slower than the rate at which thedesired set point can change. So a further feedback control may beimplemented to ensure that the (electronic) motor controller is set asaccurately as possible.

Two methods are described by which the desired grid responsive servicesof the control device of the present invention can be enabled in such aload. In a particular implementation either or both may be used.

A set point modification approach, as described above, influences thepower consumed by the device by modifying the set point or controllimits used by the PID controller to make its control decisions. Sothat, in a fridge, the lower the frequency, the lower the temperatureset point (i.e. increased energy stored), and the higher the frequency,the higher the temperature set point (i.e. decreased energy stored).More generally, the lower the frequency, the higher the internal energystored, as indicated by the Physical Variable of the Load, that thedevice aims to achieve.

FIG. 6A shows a block diagram outlining the proposed control device. Asfor a conventional controller, a manual input is used to define thenormal set point for the PID. For this controller, this sets the targetinternal energy level that will apply when the actual frequency is thesame as the central frequency. That is, it will apply when no furthercontrol over the frequency is necessary.

In this controller, a Set Point Adjusting Frequency Function feeds anadjustment to the Set Point Controller. This signal is scaled such that:when it is at its maximum positive value, the internal energy level setpoint is set to the highest permitted value; when it is zero, theinternal energy level set point is set to the manual control; and whenit is at its maximum negative value the energy level set point is set tothe lowest permitted value.

The Set Point Adjusting Frequency Function has two inputs:

1. The central frequency, derived as described above.

2. The current value of the sensed frequency.

At its simplest, the Set Point Adjusting Frequency Function may operateby comparing the two frequencies, multiplying this by a parameter, andfeeding the result as input to the Set Point Controller.

A flaw in this simple approach concerns the possibility that, if theparameters of the PID and the function (or simple multiplier) to relatethe frequency change to the change in set point were not specificallytuned for the specific circumstances of the specific grid, then there isthe possibility that the population of fridges will over orunderestimate the change on output necessary to achieve stability. Incorrecting this change, the devices could cause the frequency tooscillate.

Such oscillation (which arises from loss of what is known as SmallSignal Stability) does sometimes occur in existing grids, and, if notdetected early and corrected, can have severe consequences. Whendetected, the normal method of correction is to reconfigure the grid andgeneration so the particular frequency of oscillation is no longerresonant (a fairly hit and miss approach). It can also be resolved byreturning some of the controllers of the large gensets that participatein the oscillation. Analyzing grids to detecting and correct andreturning control is demanding of information and computationalcapability.

However, future grids, with very large numbers of grid responsivecontrol devices according to the present invention, cannot so easily bedeliberately reconfigured (it can happen accidentally as theoscillations trigger failures!)

Hence it is important to include in the automatic control system anelement of diversity in the sensitivity of response among the populationof devices. With such diversity, there is a smooth progression ofresponse from the most sensitive devices to the least, so making thechange in load monotonic with increasing departure from nominalfrequency.

The achievement of this diversity is described below by incorporating aprobability element to the set point control.

The controller uses two random numbers, chosen as described above, onefor low frequency, and one for high frequency.

If the current frequency is below the central frequency, then the setpoint adjusting function will:

1. Derive a negative value of the frequency difference (e.g. by currentfrequency−central frequency).

2. Make this value proportionate to the range over which the controllerwill operate (Minimum frequency to Nominal frequency)

3. Multiply this value by the low frequency random number.

4. Multiply the result by a sensitivity parameter defining thesensitivity of the system.

5. Feed the result to the Set Point controller, which will use this toadjust the set point and reduce the energy level it seeks.

If the actual frequency is above the central frequency, the procedure issimilar, but uses the high frequency random number, and may use adifferent sensitivity parameter.

The sensitivity parameter will be set in the light of expected gridbehavior, and may be adjusted in the light of the experience of thedevice in use.

An alternative to set point modification for a PID controller is anoutput responsive PID controller which controller adjusts the normaloutput of the PID controller to modify the actual energy consumed by thedevice according to the frequency.

With reference to FIG. 6B, the output of the PID controller is used bythe motor power controller to increase or reduce the power consumed bythe motor.

If the central frequency is the same as the actual frequency, thebehavior of the motor power controller continues to function as normalto keep the control variable within the central limits.

FIG. 6B shows a block diagram outlining the operation of such a controldevice for use with a PID controlled load.

If the central frequency and the actual frequency are different, thenthe increase or reduction in the power level of the motor is modified bythe signal from an output adjusting frequency function. With both thesesignals normalized to reflect the range over which the devices operate,the four possible control actions are each discussed:

1. If the PID controller signal is for an increase in the motor powerlevel, and the actual frequency is above that of the central frequency.The desired of both control signals are in the same direction. In thiscase the output adjusting frequency function will enlarge the increasein power level sought by the PID controller. The calculation will be:

adjusted power output level increase=PID output power levelincrease+(PID output power level increase*high frequency randomnumber*high frequency increase parameter*(actual frequency−centralfrequency)).

2. If the PID controller signal is for an increase in the motor powerlevel and the actual frequency is below that of the central frequency.In this case the desires of the two control signals are in conflict. Inthis case the output adjusting frequency function will reduce theincrease in power level sought by the PID controller. The calculationwill be:

adjusted power output level increase=PID output power levelincrease−(PID output power level increase*low frequency randomnumber*low frequency reduction parameter*(central frequency−actualfrequency)).

3. If the PID controller signal is for a reduction in the motor powerlevel, and the actual frequency is below the central frequency. Thedesires of both control signals are in the same direction. In this casethe adjusted output adjusting frequency function will enlarge theincrease in power level sought by the PID controller. The calculationwill be:

adjusted power output level reduction=PID output power levelreduction+(PID output power level reduction*low frequency randomnumber*low frequency reduction parameter*(actual frequency−centralfrequency)).

4. The PID controller signal is for a reduction in the motor powerlevel, and the actual frequency is above the central frequency. In thiscase the desires of the two control signals are in conflict. In thiscase the adjusted output adjusting frequency function will reduce thereduction in power level sought by the PID controller. The calculationwill be:

adjusted power output level reduction=PID output power levelreduction−(PID output power level reduction*high frequency randomnumber*high frequency reduction parameter*(central frequency−actualfrequency)).

The four parameters: high frequency increase parameter, low frequencyincrease parameter, low frequency reduction parameter, and highfrequency reduction parameter are set in the light of the desired gridresponse, and may be adjusted by the controller in the light of actualgrid experience.

There are many examples of loads having intermittent or variable energyconsumption in order to control a variable within central limits.Further, there are many devices that can benefit if they operate tolonger term cycles than those discussed up to now. One example from thewater industry is that of “reservoir profiling”. This is used when thereare, for example, water reservoirs that have capacity to meet theirneeds for a period of a day or so, or for long enough to span at leastone “off-peak” pricing period.

In such circumstances, it is possible to let the reservoir empty belowthe preferred level when demand for electricity is high, and replenishit when the cost of electricity is lower. So, for example, during themorning peak demand period of electricity, which also corresponds to themorning peak period for water demand, cost savings are possible bypostponing the replenishment of the reservoir until electricity demandis lower.

Yet the intermittent nature of reservoir replenishment makes it an idealcandidate for use with a grid responsive control device.

The present example control device makes use of a price parameter toprovide grid responsive control. The current price of electricity is,like frequency, also representative of the balance of generation andload on the grid.

The detection and use of a real time electricity price is discussed inGB 2407947.

The price is then used within a central limits or set point controllerto adjust the central limits of the physical variable f the load.

The principle is that, as the price rises, the limits (or set point) forthe internal energy store are lowered, and, as the price falls, thelimits (or set point) for the internal energy store are raised.

A simple, proportionate control, with the limits chosen to beproportionate to price is used.

A refinement of this is to have the price modify the “rate of change” ofthe limits. So that, if the price is high, or above a threshold set bythose you pay, then the rate at which the limits (of internal energy)are reduced is increased. The limits are prevented from passing extremesset by operational and safety requirements.

Similarly, if the price is low, or below a threshold set by those whopay it, then the rate at which the limits (of internal energy) areincreased is itself increased.

The ideal tuning for this is to enable a population of such loads to beable to provide some of both high frequency and low frequency responseat all times, but also to benefit from the longer term storage byminimizing the cost of the electricity.

The present invention also provides a black start assistance feature,which allows the energy store loads to provide grid responsive behaviorduring black starts, after a blackout has occurred. As previouslymentioned, the grid is particularly sensitive at this time and theprovision of grid frequency response loads is necessary to ensure gridstabilization at this most important of points and also to speed up therecovery of the grid.

Thus, in accordance with a fifth aspect, the present invention providesa control device for controlling an energy consumption of a load on anelectricity grid, said control device comprising:

means for delaying the starting of energy consumption of said load by arandomly generated amount of time after power is initially provided tothe control device.

A corresponding method for the fifth aspect is provided in a sixthaspect of the present invention.

In accordance with a seventh aspect, the present invention provides acontrol device for controlling an energy consumption of a load on anelectricity grid to maintain a physical variable of the load withinupper and lower limits, said control device comprising:

means for sensing the physical variable of the load;

means for providing the upper and lower limits of the sensed physicalvariable of the load; and

means for increasing the upper and/or lower limit of the sensed physicalvariable at a rate less than a maximum energy consumption of the loadafter power is initially provided to the control device.

A corresponding method for the seventh aspect is provided by an eighthaspect of the present invention.

The features of the aspects of the invention associated with the blackstart mode are combinable to provide a particularly advantageous controldevice. They may be used with grid responsive control devices of theprior art or with the grid responsive control devise hereinbeforedescribed and, particularly combinable with the previously set-outaspects and preferred aspects of the invention. The black startassistance (BSA) aspects of the invention will now be described in moredetail.

When a load is powered off, this could be due to a power cut orblackout. The control device of the present invention is adapted torecognize this possibility.

In such circumstances the grid may be delicate, and it is desirable forthe device: 1), to start providing both high and low frequency responseas soon as possible; 2), to avoid behavior synchronized with other gridresponsive control devices; and 3), to re-establish the sensed physicalvariable of the energy store load within its maximum and minimum limits.Since, however, a blackout could already have moved the sensed physicalvariable for the load outside of its control limits, a slight delay ofthe time to re-establish the load into its preferred operating rangewill generally have a lower priority than keeping the recovering gridstable.

The control device of the present invention offers a Black StartAssistance (BSA) mode upon power up to aid a recovering grid duringreconnection of load.

In one aspect of the BSA mode, the grid responsive control devicedetermines a random delay before starting. This delay is both to preventa peak load arising upon restoration of power, due to all the loadsswitching on as soon as the cut portion of the grid is reconnected, andto minimize the synchronization (maximize the diversity) of the controldevices as soon as possible. The random delay in starting-up afterre-connection in black start mode provides a gradual increase in load onthe grid after blackout.

Upon re-connection, a conventional refrigerator will set a 100 percenton duty cycle for the energy store load until the sensed physicalvariable of the load reaches its maximum control limit (ymax) and willthen shut-off immediately. In a second advantageous aspect of thepresent invention, however, response is provided by the load even whenthe load is being operated at an accelerated rate to re-establish theload within its preferred operating parameters.

According to this second advantageous aspect of BSA, the load is rampedup to its proper operating condition, i.e. when the sensed physicalvariable is within the load's control limits for the variable, with someduty cycle maintained. The provision of a duty cycle during this rampingup process allows some response to be provided, thereby aiding blackstart. In order to accelerate the load to its proper operatingcondition, the energy store load's limits for the sensed physicalvariable must be increased. Thus, the duty cycle is adapted such thatthe device will operate to a longer on portion than for normaloperation. The load is controlled, though, so it does still retain aduty cycle. One example method for achieving this is as follows.

The first step is to choose a time over which the device will reach itsproper operation. This would be some factor (greater than 1) of the timethat the load would reach this proper operation if it was notinterrupted. This factor will provide periods of no load during theblack start process. In this way, the load can both switch off in lightof a low frequency and on in light of a high frequency. Thus, the loadis able to provide response during the ramping up of the load's controllimits. This factor could, for example, be the ratio of the expectedoverall cycle time of the load to the expected on portion of that cycle.

In the case of a refrigerator currently at ambient temperature becauseof a recent blackout, the normal, 100 percent on duty cycle time forreaching its maximum temperature limits, say 0 degrees Celsius, could be30 minutes. Using a factor of two, the time for increasing the load toits normal operating temperature range will be 60 minutes.

The factor chosen can be altered by a randomization function toencourage further diversification of the load control devices.

The next step is to assess the expected on time for restoration ofnormal operation. One way to estimate this is to extrapolate from thenormal temperature change for a unit of on time of the load to determinehow long the load will need to be on starting from the current energystore level. If necessary, this estimation of the expected on time canbe made more sophisticated than a linear extrapolation.

A rate of change of the target energy store level in view of how longthe device will need to be on for and in view of the period chosen forrestoration to the desired level can then be determined.

After the random delay has passed, the low energy limit is set to thecurrent value of the sensed variable and the upper energy limit isdefined to be a normal amount of offset from the lower limit. The loadis started and moved to normal grid responsive operation.

The limits are incremented according to the chosen rate of change of theenergy store level.

An overview of the operation of the grid responsive behaviorincorporating preferred embodiments of all aspects of the invention in asingle system combined, with reference to FIGS. 4 and 5 will now begiven.

As shown in FIG. 5, the grid responsive controller is preferablyintegrated with a load for drawing energy from the grid. When the loadis first plugged into or connected to the grid, the responsive loadcontrol device is adapted to determine the current frequency of thegrid. This frequency measurement is performed periodically based upon acentral processor clock cycle or some other processing cycle of theresponsive control device, or a predetermined number of such cycles.These consecutive frequency readings will be accumulated so as tocalculate the central frequency of the grid, amongst other uses, and arecritical to the operation of the grid responsive control device of thepresent invention. Apart from the frequency measurement, the gridresponsive control device also requires a physical variable to be sensedfrom the load.

FIG. 4 shows a representation of various states and state transitions inwhich the responsive control device can operate. As can be seen fromFIG. 4, the grid responsive control device preferably starts up in ablack start assistance mode, as described above. In this way, all theloads recently connected to the grid will provide grid responsivebehavior from the beginning, which, as already described above, isespecially useful after a blackout.

As part of the black start assistance features offered by the presentinvention, the control device could be provided with an attended restartactuator (as shown in FIG. 5), which results in the sensed variable ofthe load being brought within normal control limits as soon as possibleif actuated. Thus, if the attended restart control is activated, thenthe black start assistance mode is overridden and the load is operatedat maximum energy consumption until the sensed physical variable isprovided within its control limits. This feature is useful as often theload is simply being switched on for the first time or perhaps afterbeing serviced. In these circumstances, the grid is relatively stableand on devices being operated without response for a brief period duringstart-up is inconsequential in terms of grid stability.

The attended restart actuator could be a button provided on the load.The button should be fitted where a service engineer would be aware ofit, but where it would be inconvenient for a technically aware loadowner to press. If the button was such that many load owners were awareof the attended restart button, then the black start assistance functionof the grid responsive control device of the present invention could beoverridden.

Once the sensed variable of the load is within its specified controllimits, the state of the grid will be determined, so as to derive themode of operation for the control device. The state of the grid isdetermined from the h function defined above and a measured gridfrequency as shown in FIG. 5. As previously stated and as shown in FIG.4, the grid can be in a high or low crisis state, a high or lowfrequency stress state or a normal state, depending on the value of thefunction h.

A general principle of the responsive control device of the presentinvention is that the maximum and minimum limits for the sensed physicalvariable (ymax, ymin) is dependent upon the mode of operation of thecontrol device, as outlined below.

During black start assistance mode, the current limits for the sensedphysical variable are set around the value of the sensed physicalvariable measured upon initial power-up of the load. This setting of theinitial Black Start Assistance limits for the physical variable is shownin FIG. 5. These limits are incremented at a predetermined rate untilthe normal limits for proper operation of the load are reached, asdescribed more fully above. It is an advantageous feature of theinvention that the predetermined rate of limit increment provides forthe device to have some duty cycle. Having a duty cycle will allow theload to provide response, rather than the alternative of having the loadcontinuously on.

The increment of the limits during BSA mode is always performed unless:a period of low frequency stress or crisis is determined, in which casethe limits are frozen; or a period of high frequency stress or crisis isdetermined when the rate of increment is increased. During a lowfrequency stress or crisis state, there is too much load on the grid,and so continuing to increase the energy consumption of the responsiveloads is not appropriate. During a high frequency stress or crisisstate, there is too much generation, so it will be beneficial to thegrid to increase the rate of increment.

During lower frequency crisis, the limits of the physical variable ofthe load are decremented, until they reach a minimum energy state (y=0).The rate of decrement is chosen to be approximately half the on runningtime of the load, so some response will remain as the limits are reducedtowards zero.

During low frequency stress, the current limits of the load, as definedby the set point of the load, are frozen so as to prevent adjustment ofthe set point by the user. The exception to this freezing of the limitsis in the case of recovery from low frequency crisis, during which timethe limits are incremented to bring them back towards their value beforethe crisis state was entered.

Once the normal mode of operation after BSA has been reached, the limitsof the sensed physical variable are preferably controlled depending uponwhether the grid is facing high frequency stress or crisis or lowfrequency stress or crisis. During high frequency stress or crisis, offdevices are preferably turned on in order to take up the excessgeneration. Thus, the value of ymax is preferably increased such that ondevices will remain on for a longer period of time and previously offdevices that have just been switched on because of the high frequencyremain on for an extended period of time as well. During a low frequencystress or crisis, the opposite is true, and there is too much load onthe grid. This means that the lower limit of the sensed physicalvariable (ymin) is decreased to ensure off devices remain off for anextra amount of time.

During high frequency crisis, the limits are incremented until theyreach a maximum energy store level (y=1). The increments are chosen toapproximately double the on portion of the duty cycle of the load, so asto reduce the energy store level, but still maintain some response.

During high frequency stress, the minimum and maximum limits for thesensed physical variable are frozen for the same reason that they arefrozen during a period of low frequency stress—to prevent set pointadjustment. An exception to these limits being prevented from beingchanged occurs when the grid is in recovery from high or low frequencycrisis, when the limits are moved in small steps until they have becomethose used before the grid entered a crisis state.

The increment of the limits during high frequency crisis or black startassistance mode of operation of the grid responsive control device andthe decrement of the limits during low frequency crisis are illustrativeof another novel and advantageous feature of the present invention overthe prior art. According to the present invention, even during such raregrid events, some grid response behavior is still given. This responseis particularly beneficial during these grid states if grid stability isto be recovered.

The dashed lines in FIG. 4 show illegal transitions which representstrange behavior of the grid. For example moving directly from a lowfrequency crisis state to a high frequency crisis state should notoccur. In general if such a transition does happen, an intermediatestate is chosen by the control device to make the state transition ofthe grid responsive control device less abrupt.

While the minimum and maximum limits of the sensed variable are changeddepending upon the mode of operation of the device, the determination ofthe trigger frequency is as previously described. The only differencebeing that the average temperature level in a population of such deviceswill be extended over a larger temperature range depending on the modeof operation. Thus, in a crisis mode, the load's variable limit (ymin orymax) will be extended as compared to the limits during normaloperation. This will result in the population of the devices providingresponse over an extended range of the physical variable of the load.

With reference to FIG. 5, once the device has started-up in black startmode and once the grid status has been determined, the target andtrigger frequencies will be calculated using the adjusted ymax and/orymin, which are adjusted depending on the grid status, and the currentvalue of the physical variable of the load, as sensed. Having sensed thegrid frequency and the physical variable of the load and having obtaineda value of the sensed frequency to trigger the load on or off, adecision can be as to whether to switch the device. This decision ismade by comparing the sensed frequency to the trigger frequency and bycomparing the sensed variable of the load to the current limits for theload's sensed variable.

Further steps are also shown in FIG. 5. These steps involve capturingdata concerning device operation and using this data to tune theoperation of the device. This capturing and tuning has already beendiscussed above with respect to the provision of the grid frequencylimits and their adjustment depending upon experience of the grid.Further possibilities for tuning the device are discussed below. Thetuned variables could potentially be stored and re-used advantageously.

FIG. 5 also shows the possibility of communicating data captured andthis is discussed below.

The present invention also encompasses a grid responsive control deviceas discussed above with certain modifications. These modifications areoptional features that may offer particular improvements to the controldevice already discussed.

The control device of the present invention aims to prevent rapidswitching of the energy store loads, but there may still be certain gridconditions that result in an excessive switching rate, particularly whenthe grid is under stress. Such rapid switching rates may, in the case ofa refrigerator for example, make its compressor ineffective as well asdamaging it. The ineffectiveness of the compressor may result from aminimum time needed for internal pressure in the compressor to dissipateafter being switched off. If it is switched on again before this hashappened, the high pressure in the compressor cannot be overcome (itneeds an extra push from the inertia of a running pump), so it willstall. This can create a high electrical load, dissipated as heat,putting the whole device at risk. Refrigerators usually have stall orthermal detectors which disconnect power and so protect the device fromthis damage.

The responsive control device of the present invention may include ahysterisis feature, such that an on or off state is maintained for aminimum period, and this can be set to suit the device. This hysterisisfeature is a backup, as the trigger frequency trajectory being biased tominimize switching should normally prevent any rapid switching. It willonly be in the most extreme grid conditions that the switching rate willbecome excessive and the hysterisis feature is required.

The grid responsive control device of the present invention should becapable of operating without any external input, apart from thefrequency and the sensed variable. The grid responsive control deviceshould also be autonomous over the whole life of the energy store.

In order to achieve such autonomy requirements, the grid responsivecontrol device of the present invention is preferably adapted to detectthe nominal frequency (and this step is shown in FIG. 5) of the griditself. As described above, it is important for the present invention tobe aware of the nominal frequency so that the control device can biasits grid response behavior so as to urge the system frequency towardsthe nominal frequency.

There are other grid particular settings which the present inventionmakes use of, and which, the grid responsive control device should beable to ascertain from experience of the grid to which it is connectedand not from additional inputs. One other example is the detection ofthe upper and lower frequency limits, as described above.

In view of the above requirements of the present invention, the gridresponsive control device is adapted to determine the nominal frequencyafter taking a series of measurements. For each of these measurements, aset of “standard nominal frequencies” stored in a memory of the controldevice are interrogated and the closest standard nominal frequency tothe grid frequency measurement is taken as the standard frequency forthat measurement. Once the same standard frequency has been determinedfrom a consecutive number of frequency measurements, the valuedetermined is chosen as the nominal frequency of the grid. Theresponsive control of the present invention is, therefore, required tokeep a list of possible normal frequencies, such as 50 hertz, 60 hertzand 400 hertz.

The control device of the present invention may also be configured to beaware of certain pre-established periods of time, which are employed insaving any current settings learned from the grid. Any of these gridexperience determined parameters can be saved in long term non-volatilememory at the end of an appropriate time period. In this way, keyfeatures of the grid behavior can be recorded onto long term memory.

The ability to store data and update this data as the device learns fromthe grid's behavior and the load's behavior is an important feature ofthe present invention (and is shown In FIG. 5) as it is very possiblethat a particular load could be moved between grids. For example, inDenmark the load will not even need to be moved internationally tochange grids. Each grid will behave differently and the grid responsivecontrol device will need to react to this and adapt accordingly.

The control device will also need to tune itself to the grid's behaviorbecause it is possible that this behavior could change with time,particularly as more and more of the grid responsive control devices areapplied to the energy store loads on the grid. The self-tuning, however,needs to be performed carefully as it would not be helpful if, forexample, a sustained period of grid, instability caused self-tuning thatdamaged the device's ability to respond during a rare crisis.

The responsive control devices may also need to tune their parameters totake into account the behavior of the load. For example, a very fullrefrigerator does not behave in quite the same way as a nearly emptyone.

The possibilities for self-tuning are presently envisaged to includeoptimization taking account of variation in the expected duty cycletime, optimization of the maximum and minimum frequency limits in lightof grid experience (as discussed above) and optimizing the use ofhistorical frequency behaviors within the adaptation parameters.

If a load is recovering from a blackout, it will be desirable to retainany tuned parameters achieved before the blackout. This requires storingof the tuned parameters and other captured data, as shown in FIG. 5. Thecontrol device, however, also needs to take into account that the devicecould be being switched on for the very first time and there are not anypreviously tuned parameters to recover. The general principle to whichthe grid responsive control devices will be operated is that the devicewill aim to recover earlier tuning, unless the device has beendisconnected for so long that it cannot be a blackout, or the gridnominal frequency has changed.

A hardware feature could be used to determine whether the device hasbeen disconnected for longer than a blackout, such as a leaky capacitor,which, when this is discharged, suggests the load is in a y=0 state.

Thus, the controller is provided with some means of determining whetherthe load was switched off because of a blackout or simply because theuser had switched it off. In both cases, recovery of previously tunedparameters is appropriate. If, however, the load is being switched onfor the first time, or is likely to have been moved between grids, then,loading of previously determined parameters from memory will not beperformed.

Recovery from a blackout also realizes the possibility of all of thedata capture periods of a population of the loads connected to the gridbecoming synchronized. None of the currently envisaged processes dependscritically upon diversified periods, but the possibility of rapid changein grid behavior from simultaneous identical self-tuning is removed ifthey are. Thus, upon initial switch on, the grid responsive controldevices of the present invention are preferably adapted to choose arandom time for any periods that the device makes use of.

The responsive control device of the present invention often makes useof the time which the device is expected to be on, and the time whichthe device is expected to be off, for example in determining the rate ofincrement or decrement of the sensed variable control limits duringblack start assistance or high or low crisis operation. The time thedevice is expected to be on or off is the time the load is expected totake in moving from one sensed variable value to another. Tuning of thisexpectation time is possible based on experience of how the sensedvariable of the load reacts to a particular energy consumption level.

One way of optimizing the load's response to energy consumption is asfollows. After each change of state, i.e. switching from an on status toan off status or vice visa, it is possible to note how long the load hasrun, and the extent of change of the sensed variable in that time. Forestimating an expected on time or off time for a particular variablechange, these noted values can be extrapolated. How the sensed variablewill change with on/off time depends upon its current use, e.g. how fullit is and how often it has been opened. The expected on or off timecalculations could be performed at each switching point, for example.

The responsive control device may also make use of a prediction of howlong it will be in an on state or an off state. This can be determinedfrom a moving average of the actual times of previous states.

It is clear from inspection of frequency charts that different gridshave real differences in their frequency behavior. The range over whichthe frequency varies is one important aspect, but there are also moresubtle differences such as its tendency to fluctuate, the usual lengthof excursions above nominal, etc. It is possible that these features canbe used to modify some of the parameters, such as the parametersadjusting the rate at which frequency limits narrow. Thus it isimportant for the responsive control device of the present invention tocapture information on the behavior of the grid frequency, particularlyat the end of natural periods, such as a frequency excursion, and of aparticular grid state (normal, stressed, or crisis), the end ofparticular state of the load (on or off) or the end of an operationcycle (one cycle of the processor controlling the timings of the majorfunctions of the control device). All of the information captured couldbe used for the input in tuning the responsive control device so as tooptimize operation with respect to the grid to which it is connected.

The responsive control device of the present invention may also includesome form of communication means, and a communication step is shown inFIG. 5, so that the data collected can be transferred. The transferenceof data will normally be provided by maintenance personnel. Thecommunication means may also be available such that the software of theresponsive control device or the grid parameters may be updated upon amaintenance visit. The communication means will also make it possible tocapture measurements of the grid behavior during the working life of theload and also the loads contribution to the grid. Thus, some measure ofload's value to the grid can be determined.

1-51. (canceled)
 52. A control device for controlling an energyconsumption of a load operably connected to an electricity grid,characterized in that said control device comprises: means for sensing avalue of a physical variable of the grid, said physical variable varyingdepending upon a relationship between electricity generation and aloading on the grid; means for sensing a value of a physical variable ofthe load, said physical variable of the load being representative ofenergy stored by the load; means for varying an energy consumption ofsaid load when a value of said physical variable of the grid reaches atrigger value; and means for determining the trigger value, saiddetermining of the trigger value being dependent upon said sensedphysical variable of the load.
 53. A control device as claimed in claim52, wherein said means for varying comprises means for operablycomparing said trigger value with the current sensed physical variableof the grid.
 54. A control device as claimed in claim 53, wherein saidmeans for varying the energy consumption of the load is configured tovary in operation the energy consumption of the load for maintaining thesensed physical variable of the load within central limits, and isfurther configured to vary the energy consumption when said value of thephysical variable of the grid reaches the trigger value.
 55. A controldevice as claimed in claim 54, wherein said means for determining thetrigger value is configured to determine the trigger value dependingupon the sensed physical variable of the load and is operable to providea limiting control for reducing the rate of variation of the energyconsumption of the load, or wherein said means for determining saidtrigger value comprises a function which is operable to return saidtrigger value depending upon said physical variable of the load, saidfunction defining a trigger value profile varying with said physicalvariable of the load, wherein said profile is such that the morerecently the energy consumption of the load has varied, the further thetrigger value is from a central value of the physical variable of thegrid.
 56. A control device as claimed in claim 55, wherein said meansfor varying is configured to vary said energy consumption by switchingthe energy consumption of the load between a first state of increasingthe energy stored by the load, and a second state of the decreasing theenergy stored in the load.
 57. A control device as claimed in claim 56,wherein said control device further comprises: means for determining acentral value of the physical variable of the grid from said values ofthe physical variable of the grid; and said means for varying the energyconsumption of said load is operable to vary said energy consumptiondepending upon said central value.
 58. A control device as claimed inclaim 57, wherein said device further includes: means for varying theenergy consumption of said load when a current value of said sensedphysical variable of the grid reaches a trigger value; and means fordetermining said trigger value, wherein said trigger value is operablydetermined depending upon said central value.
 59. A control device asclaimed in claim 58, wherein said means for determining said triggervalue comprises a function for randomly providing said trigger valuebetween a determined upper or lower value of the physical variable ofthe grid and said central value.
 60. A control device as claimed inclaim 58, wherein said means for sensing a physical variable of the loadis operable to sense a value of physical variable which isrepresentative of the energy stored in the load, and wherein saiddetermining of the trigger value is dependent upon said sensed physicalvariable of the load.
 61. A control device as claimed in claim 56,wherein said sensed physical variable of the grid is a sensed frequencyof the grid.
 62. A method of controlling an energy consumption of a loadoperably connected to an electricity grid, characterized in that saidmethod comprises: sensing a value of a physical variable of the grid,wherein said physical variable of the grid depends on a relationshipbetween electricity generation and loading on the grid; sensing a valueof a physical variable of the load, wherein said physical variable ofthe load is representative of the energy stored in the load; varying theenergy consumption of the said load when a value of said physicalvariable of the grid reaches a trigger value; and determining thetrigger value, wherein said trigger value depends upon said sensedphysical variable of the load.
 63. A method as claimed in claim 62,wherein said varying comprises comparing said trigger value with thecurrent sensed physical variable of the grid.
 64. A method as claimed inclaim 63, wherein the varying of the energy consumption of the loadcomprises varying the energy consumption of the load for maintaining thesensed physical variable of the load within control limits, and furthervarying the energy consumption when said value of the physical variableof the grid reaches the trigger value.
 65. A method as claimed in claim64, wherein said determining of the trigger value comprises determiningthe trigger value depending upon the sensed physical variable of theload within control limits for reducing a rate of variation of theenergy consumption of the load, or wherein said determining said triggervalue comprises returning said trigger value depending upon saidphysical variable of the load, by defining a trigger value profile whichvaries with said physical variable of the load, wherein said profile issuch that the more recently the energy consumption of the load hasvaried the further the trigger value is from a central value of thephysical variable of the grid.
 66. A method as claimed in claim 65,wherein said sensed physical variable of the grid is a sensed frequencyof the grid.
 67. A method as claimed in claim 66, wherein said varyingcomprises varying said energy consumption by switching the energyconsumption between a first state of increasing the energy stored in theload and a second state of decreasing the energy stored in the load.